Fix bug #12537 UNION produces longtext instead of varchar

Item::tmp_table_field_from_field_type() and create_tmp_field_from_item()
was converting string field to blob depending on byte-wise length instead of
character length, which results in converting valid varchar string with
length == 86 to longtext.

Made that functions above take into account max width of character when
converting string fields to blobs.

mysql-test/r/create.result

@@ -580,6 +580,13 @@ ERROR 42000: Incorrect database name 'xyz'
 create table t1(t1.name int);
 create table t2(test.t2.name int);
 drop table t1,t2;
+CREATE TABLE t1 (f1 VARCHAR(255) CHARACTER SET utf8);
+CREATE TABLE t2 AS SELECT LEFT(f1,86) AS f2 FROM t1 UNION SELECT LEFT(f1,86)
+AS f2 FROM t1;
+DESC t2;
+Field	Type	Null	Key	Default	Extra
+f2	varchar(86)	YES		NULL	
+DROP TABLE t1,t2;
 create database mysqltest;
 use mysqltest;
 drop database mysqltest;

mysql-test/t/create.test

@@ -492,6 +492,15 @@ create table t1(t1.name int);
 create table t2(test.t2.name int);
 drop table t1,t2;
 
+#
+# Bug #12537: UNION produces longtext instead of varchar
+#
+CREATE TABLE t1 (f1 VARCHAR(255) CHARACTER SET utf8);
+CREATE TABLE t2 AS SELECT LEFT(f1,86) AS f2 FROM t1 UNION SELECT LEFT(f1,86)
+AS f2 FROM t1;
+DESC t2;
+DROP TABLE t1,t2;
+
 #
 # Bug#11028: Crash on create table like
 #

sql/item.cc

@@ -2022,12 +2022,14 @@ Field *Item::tmp_table_field_from_field_type(TABLE *table)
   case MYSQL_TYPE_ENUM:
   case MYSQL_TYPE_SET:
   case MYSQL_TYPE_VAR_STRING:
-    if (max_length > 255)
+    DBUG_ASSERT(collation.collation);
+    if (max_length/collation.collation->mbmaxlen > 255)
       break;					// If blob
     return new Field_varstring(max_length, maybe_null, name, table,
 			       collation.collation);
   case MYSQL_TYPE_STRING:
-    if (max_length > 255)			// If blob
+    DBUG_ASSERT(collation.collation);
+    if (max_length/collation.collation->mbmaxlen > 255)		// If blob
       break;
     return new Field_string(max_length, maybe_null, name, table,
 			    collation.collation);

sql/sql_select.cc

@@ -4899,7 +4899,8 @@ static Field* create_tmp_field_from_item(THD *thd, Item *item, TABLE *table,
 				   item->name, table, item->unsigned_flag);
     break;
   case STRING_RESULT:
-    if (item->max_length > 255)
+    DBUG_ASSERT(item->collation.collation);
+    if (item->max_length/item->collation.collation->mbmaxlen > 255)
     {
       if (convert_blob_length)
         new_field= new Field_varstring(convert_blob_length, maybe_null,