Fix file descriptor leak on error in read_igmp()

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

ip/ipmaddr.c

@@ -128,8 +128,10 @@ void read_igmp(struct ma_info **result_p)
 	if (!fp)
 		return;
 	memset(&m, 0, sizeof(m));
-	if (!fgets(buf, sizeof(buf), fp))
+	if (!fgets(buf, sizeof(buf), fp)) {
+		fclose(fp);
 		return;
+	}
 
 	m.addr.family = AF_INET;
 	m.addr.bitlen = 32;