MDEV-27195 SIGSEGV in Table_scope_and_contents_source_st::vers_check_system_fields

The old code erroneously used default_charset_info to compare field names.
default_charset_info can point to any arbitrary collation,
including ucs2*, utf16*, utf32*, including those that do not
support strcasecmp().

my_charset_utf8mb4_unicode_ci, which is used in this scenario:

CREATE TABLE t1 ENGINE=InnoDB WITH SYSTEM VERSIONING AS SELECT 0;

does not support strcasecmp().

Fixing the code to use Lex_ident::streq(), which uses
system_charset_info instead of default_charset_info.

mysql-test/main/ctype_utf8mb4_unicode_ci_def.opt

+--character-set-server=utf8mb4,latin1 --collation-server=utf8mb4_unicode_ci

mysql-test/main/ctype_utf8mb4_unicode_ci_def.result

+#
+# Start of 10.3 tests
+#
+#
+# MDEV-27195 SIGSEGV in Table_scope_and_contents_source_st::vers_check_system_fields
+#
+CREATE TABLE t1 ENGINE=MyISAM WITH SYSTEM VERSIONING AS SELECT 0;
+DROP TABLE t1;
+#
+# End of 10.3 tests
+#

mysql-test/main/ctype_utf8mb4_unicode_ci_def.test

+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-27195 SIGSEGV in Table_scope_and_contents_source_st::vers_check_system_fields
+--echo #
+
+CREATE TABLE t1 ENGINE=MyISAM WITH SYSTEM VERSIONING AS SELECT 0;
+DROP TABLE t1;
+
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/handler.cc

@@ -7264,8 +7264,7 @@ bool Table_scope_and_contents_source_st::vers_check_system_fields(
       {
         List_iterator<Create_field> dup_it(alter_info->create_list);
         for (Create_field *dup= dup_it++; !is_dup && dup != f; dup= dup_it++)
-          is_dup= my_strcasecmp(default_charset_info,
-                                dup->field_name.str, f->field_name.str) == 0;
+          is_dup= Lex_ident(dup->field_name).streq(f->field_name);
       }
 
       if (!(f->flags & VERS_UPDATE_UNVERSIONED_FLAG) && !is_dup)