MDEV-7003 test-alter-table crashes debug build due to double free of plugin

correct the buffer boundary check

sql/table.cc

@@ -946,10 +946,10 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
       size_t length= *extra2++;
       if (!length)
       {
-        if (extra2 + 258 >= e2end)
+        if (extra2 + 2 >= e2end)
           goto err;
         length= uint2korr(extra2);
-        extra2+=2;
+        extra2+= 2;
         if (length < 256)
           goto err;
       }

storage/tokudb/mysql-test/tokudb_mariadb/t/229cols.test

+#
+# MDEV-6106 Cannot create a table with 229 or greater columns in TokuDB
+# MDEV-7003 test-alter-table crashes debug build due to double free of plugin
+#
 create table t1(
 	col1 varchar(40),
 	col2 varchar(40),
@@ -227,8 +231,244 @@ create table t1(
 	col226 varchar(40),
 	col227 varchar(40),
 	col228 varchar(40),
- 	col229 varchar(40) 
+ 	col229 varchar(40)
 ) engine=TokuDB;
 show create table t1;
+alter table t1 add i230 integer;
+show create table t1;
 drop table t1;
 
+create table t1(
+	col1 varchar(40),
+	col2 varchar(40),
+	col3 varchar(40),
+	col4 varchar(40),
+	col5 varchar(40),
+	col6 varchar(40),
+	col7 varchar(40),
+	col8 varchar(40),
+	col9 varchar(40),
+	col10 varchar(40),
+	col11 varchar(40),
+	col12 varchar(40),
+	col13 varchar(40),
+	col14 varchar(40),
+	col15 varchar(40),
+	col16 varchar(40),
+	col17 varchar(40),
+	col18 varchar(40),
+	col19 varchar(40),
+	col20 varchar(40),
+	col21 varchar(40),
+	col22 varchar(40),
+	col23 varchar(40),
+	col24 varchar(40),
+	col25 varchar(40),
+	col26 varchar(40),
+	col27 varchar(40),
+	col28 varchar(40),
+	col29 varchar(40),
+	col30 varchar(40),
+	col31 varchar(40),
+	col32 varchar(40),
+	col33 varchar(40),
+	col34 varchar(40),
+	col35 varchar(40),
+	col36 varchar(40),
+	col37 varchar(40),
+	col38 varchar(40),
+	col39 varchar(40),
+	col40 varchar(40),
+	col41 varchar(40),
+	col42 varchar(40),
+	col43 varchar(40),
+	col44 varchar(40),
+	col45 varchar(40),
+	col46 varchar(40),
+	col47 varchar(40),
+	col48 varchar(40),
+	col49 varchar(40),
+	col50 varchar(40),
+	col51 varchar(40),
+	col52 varchar(40),
+	col53 varchar(40),
+	col54 varchar(40),
+	col55 varchar(40),
+	col56 varchar(40),
+	col57 varchar(40),
+	col58 varchar(40),
+	col59 varchar(40),
+	col60 varchar(40),
+	col61 varchar(40),
+	col62 varchar(40),
+	col63 varchar(40),
+	col64 varchar(40),
+	col65 varchar(40),
+	col66 varchar(40),
+	col67 varchar(40),
+	col68 varchar(40),
+	col69 varchar(40),
+	col70 varchar(40),
+	col71 varchar(40),
+	col72 varchar(40),
+	col73 varchar(40),
+	col74 varchar(40),
+	col75 varchar(40),
+	col76 varchar(40),
+	col77 varchar(40),
+	col78 varchar(40),
+	col79 varchar(40),
+	col80 varchar(40),
+	col81 varchar(40),
+	col82 varchar(40),
+	col83 varchar(40),
+	col84 varchar(40),
+	col85 varchar(40),
+	col86 varchar(40),
+	col87 varchar(40),
+	col88 varchar(40),
+	col89 varchar(40),
+	col90 varchar(40),
+	col91 varchar(40),
+	col92 varchar(40),
+	col93 varchar(40),
+	col94 varchar(40),
+	col95 varchar(40),
+	col96 varchar(40),
+	col97 varchar(40),
+	col98 varchar(40),
+	col99 varchar(40),
+	col100 varchar(40),
+	col101 varchar(40),
+	col102 varchar(40),
+	col103 varchar(40),
+	col104 varchar(40),
+	col105 varchar(40),
+	col106 varchar(40),
+	col107 varchar(40),
+	col108 varchar(40),
+	col109 varchar(40),
+	col110 varchar(40),
+	col111 varchar(40),
+	col112 varchar(40),
+	col113 varchar(40),
+	col114 varchar(40),
+	col115 varchar(40),
+	col116 varchar(40),
+	col117 varchar(40),
+	col118 varchar(40),
+	col119 varchar(40),
+	col120 varchar(40),
+	col121 varchar(40),
+	col122 varchar(40),
+	col123 varchar(40),
+	col124 varchar(40),
+	col125 varchar(40),
+	col126 varchar(40),
+	col127 varchar(40),
+	col128 varchar(40),
+	col129 varchar(40),
+	col130 varchar(40),
+	col131 varchar(40),
+	col132 varchar(40),
+	col133 varchar(40),
+	col134 varchar(40),
+	col135 varchar(40),
+	col136 varchar(40),
+	col137 varchar(40),
+	col138 varchar(40),
+	col139 varchar(40),
+	col140 varchar(40),
+	col141 varchar(40),
+	col142 varchar(40),
+	col143 varchar(40),
+	col144 varchar(40),
+	col145 varchar(40),
+	col146 varchar(40),
+	col147 varchar(40),
+	col148 varchar(40),
+	col149 varchar(40),
+	col150 varchar(40),
+	col151 varchar(40),
+	col152 varchar(40),
+	col153 varchar(40),
+	col154 varchar(40),
+	col155 varchar(40),
+	col156 varchar(40),
+	col157 varchar(40),
+	col158 varchar(40),
+	col159 varchar(40),
+	col160 varchar(40),
+	col161 varchar(40),
+	col162 varchar(40),
+	col163 varchar(40),
+	col164 varchar(40),
+	col165 varchar(40),
+	col166 varchar(40),
+	col167 varchar(40),
+	col168 varchar(40),
+	col169 varchar(40),
+	col170 varchar(40),
+	col171 varchar(40),
+	col172 varchar(40),
+	col173 varchar(40),
+	col174 varchar(40),
+	col175 varchar(40),
+	col176 varchar(40),
+	col177 varchar(40),
+	col178 varchar(40),
+	col179 varchar(40),
+	col180 varchar(40),
+	col181 varchar(40),
+	col182 varchar(40),
+	col183 varchar(40),
+	col184 varchar(40),
+	col185 varchar(40),
+	col186 varchar(40),
+	col187 varchar(40),
+	col188 varchar(40),
+	col189 varchar(40),
+	col190 varchar(40),
+	col191 varchar(40),
+	col192 varchar(40),
+	col193 varchar(40),
+	col194 varchar(40),
+	col195 varchar(40),
+	col196 varchar(40),
+	col197 varchar(40),
+	col198 varchar(40),
+	col199 varchar(40),
+	col200 varchar(40),
+	col201 varchar(40),
+	col202 varchar(40),
+	col203 varchar(40),
+	col204 varchar(40),
+	col205 varchar(40),
+	col206 varchar(40),
+	col207 varchar(40),
+	col208 varchar(40),
+	col209 varchar(40),
+	col210 varchar(40),
+	col211 varchar(40),
+	col212 varchar(40),
+	col213 varchar(40),
+	col214 varchar(40),
+	col215 varchar(40),
+	col216 varchar(40),
+	col217 varchar(40),
+	col218 varchar(40),
+	col219 varchar(40),
+	col220 varchar(40),
+	col221 varchar(40),
+	col222 varchar(40),
+	col223 varchar(40),
+	col224 varchar(40),
+	col225 varchar(40),
+	col226 varchar(40),
+	col227 varchar(40),
+	col228 varchar(40),
+ 	col229 varchar(40),
+ 	col230 varchar(40)
+) engine=TokuDB;
+show create table t1;
+drop table t1;