MDEV-9225 mysql_upgrade segfault due to missing /etc/my.cnf.d

In case of missing includedir file, we would attempt to free a NULL
pointer. Make sure to guard against that.

client/mysql_upgrade.c

@@ -182,7 +182,8 @@ static const char *load_default_groups[]=
 static void free_used_memory(void)
 {
   /* Free memory allocated by 'load_defaults' */
-  free_defaults(defaults_argv);
+  if (defaults_argv)
+    free_defaults(defaults_argv);
 
   dynstr_free(&ds_args);
   dynstr_free(&conn_args);