MDEV-22035 Memory leak in main.mysqltest

The test main.mysqltest could crash or hang with
cmake -DWITH_ASAN=ON builds. The reason appears to be
a memory leak, which was found out by manually invoking

echo --replace_regex a > file
ASAN_OPTIONS=log_path=/dev/shm/asan mysqltest ... < file

and then examining the /dev/shm/asan.* file.

client/mysqltest.cc

@@ -10190,6 +10190,7 @@ void append_replace_regex(char* expr, char *expr_end, struct st_replace_regex* r
   return;
 
 err:
+  my_free(res->regex_arr.buffer);
   my_free(res);
   die("Error parsing replace_regex \"%s\"", expr);
 }