MDEV-6224 Incorrect information in file when *.frm is > 256K

Reject huge frms at CREATE TABLE, not when it - successfully written - is being opened.
Also raise the frm size limit from 256K to 512K

mysql-test/r/huge_frm-6224.result

+ERROR HY000: The definition for table `t1` is too big

mysql-test/t/huge_frm-6224.test

+#
+# MDEV-6224 Incorrect information in file when *.frm is > 256K
+#
+# verify that huge frms are rejected during creation, not on opening
+#
+--source include/have_partition.inc
+
+let $n=5646;
+let $a=create table t1 (a int) engine=myisam partition by hash(a) partitions $n (;
+dec $n;
+while ($n)
+{
+  let $a=$a partition p01234567890123456789012345678901234567890123456789012345678$n,;
+  dec $n;
+}
+
+--disable_query_log
+--error ER_TABLE_DEFINITION_TOO_BIG
+eval $a partition foo);
+

sql/share/errmsg-utf8.txt

@@ -7109,3 +7109,5 @@ ER_IT_IS_A_VIEW 42S02
         eng "'%-.192s' is a view"
 ER_SLAVE_SKIP_NOT_IN_GTID
 	eng "When using GTID, @@sql_slave_skip_counter can not be used. Instead, setting @@gtid_slave_pos explicitly can be used to skip to after a given GTID position."
+ER_TABLE_DEFINITION_TOO_BIG
+        eng "The definition for table %`s is too big"

sql/unireg.cc

@@ -211,6 +211,12 @@ LEX_CUSTRING build_frm_image(THD *thd, const char *table,
   filepos= frm.length;
   frm.length+= FRM_FORMINFO_SIZE;               // forminfo
   frm.length+= packed_fields_length(create_fields);
+
+  if (frm.length > FRM_MAX_SIZE)
+  {
+    my_error(ER_TABLE_DEFINITION_TOO_BIG, MYF(0), table);
+    DBUG_RETURN(frm);
+  }
   
   frm_ptr= (uchar*) my_malloc(frm.length, MYF(MY_WME | MY_ZEROFILL |
                                               MY_THREAD_SPECIFIC));

sql/unireg.h

@@ -203,7 +203,7 @@ LEX_CUSTRING build_frm_image(THD *thd, const char *table,
 
 #define FRM_HEADER_SIZE 64
 #define FRM_FORMINFO_SIZE 288
-#define FRM_MAX_SIZE (256*1024)
+#define FRM_MAX_SIZE (512*1024)
 
 static inline bool is_binary_frm_header(uchar *head)
 {