Fix file descriptor leak on error in iproute_flush_cache()

Detected by cppcheck.
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>

ip/iproute.c

@@ -1057,6 +1057,7 @@ static int iproute_flush_cache(void)
 
 	if ((write (flush_fd, (void *)buffer, len)) < len) {
 		fprintf (stderr, "Cannot flush routing cache\n");
+		close(flush_fd);
 		return -1;
 	}
 	close(flush_fd);