lib-storage: Fix mail_storage_set_critical() when input parameter is …

…an old internal error

This could have caused garbage in the error string.

src/lib-storage/mail-storage.c

@@ -518,15 +518,19 @@ void mail_storage_set_internal_error(struct mail_storage *storage)
 void mail_storage_set_critical(struct mail_storage *storage,
 			       const char *fmt, ...)
 {
+	char *old_error = storage->last_internal_error;
 	va_list va;
 
-	i_free(storage->last_internal_error);
 	va_start(va, fmt);
 	storage->last_internal_error = i_strdup_vprintf(fmt, va);
 	va_end(va);
 	storage->last_error_is_internal = TRUE;
 	i_error("%s", storage->last_internal_error);
 
+	/* free the old_error only after the new error is generated, because
+	   the old_error may be one of the parameters. */
+	i_free(old_error);
+
 	/* critical errors may contain sensitive data, so let user
 	   see only "Internal error" with a timestamp to make it
 	   easier to look from log files the actual error message. */