Fix file descriptor leak on error in read_mroute_list()

Detected by cppcheck.

Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
diff --git a/ip/ipmroute.c b/ip/ipmroute.c
index f033349..945727d 100644
--- a/ip/ipmroute.c
+++ b/ip/ipmroute.c
@@ -85,8 +85,10 @@
 	if (!fp)
 		return;
 
-	if (!fgets(buf, sizeof(buf), fp))
+	if (!fgets(buf, sizeof(buf), fp)) {
+		fclose(fp);
 		return;
+	}
 
 	while (fgets(buf, sizeof(buf), fp)) {
 		inet_prefix maddr, msrc;