[settings] Fix broken assertion in fetch_string_setting_copy()

Reported-by: Stefan Hajnoczi <stefanha@gmail.com>

src/core/settings.c

 				struct setting *setting,
 				char **data ) {
 	int len;
-	int check_len;
+	int check_len = 0;
 
 	len = fetch_setting_len ( settings, setting );
 	if ( len < 0 )
 	if ( ! *data )
 		return -ENOMEM;
 
-	fetch_string_setting ( settings, setting, *data, ( len + 1 ) );
+	check_len = fetch_string_setting ( settings, setting, *data,
+					   ( len + 1 ) );
 	assert ( check_len == len );
 	return len;
 }