Print a warning when an old table (with no character set stored) is opened

and the default character set is multi-byte, which will result in character
column size changes. (Bug #6913)

sql/table.cc

@@ -145,8 +145,19 @@ int openfrm(const char *name, const char *alias, uint db_stat, uint prgflag,
     outparam->table_charset=get_charset((uint) head[38],MYF(0));
     null_field_first=1;
   }
-  if (!outparam->table_charset) /* unknown charset in head[38] or pre-3.23 frm */
+  if (!outparam->table_charset)
+  {
+    /* unknown charset in head[38] or pre-3.23 frm */
+    if (use_mb(default_charset_info))
+    {
+      /* Warn that we may be changing the size of character columns */
+      sql_print_warning("'%s' had no or invalid character set, "
+                        "and default character set is multi-byte, "
+                        "so character column sizes may have changed",
+                        name);
+    }
     outparam->table_charset=default_charset_info;
+  }
   outparam->db_record_offset=1;
   if (db_create_options & HA_OPTION_LONG_BLOB_PTR)
     outparam->blob_ptr_size=portable_sizeof_char_ptr;