MDEV-11548 Reproducible server crash after the 2nd ALTER TABLE ADD FOREIGN KEY IF NOT EXISTS.

Have to use 'keyname' to check the name uniqueness.

sql/sql_table.cc

@@ -5943,7 +5943,7 @@ drop_create_field:
         while ((f_key= fk_key_it++))
         {
           if (my_strcasecmp(system_charset_info, f_key->foreign_id->str,
-                key->name.str) == 0)
+                keyname) == 0)
             goto remove_key;
         }
       }