Fix file descriptor leak on error in read_viftable()

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

ip/ipmroute.c

@@ -58,9 +58,10 @@ static void read_viftable(void)
 	if (!fp)
 		return;
 
-	if (!fgets(buf, sizeof(buf), fp))
+	if (!fgets(buf, sizeof(buf), fp)) {
+		fclose(fp);
 		return;
-
+	}
 	while (fgets(buf, sizeof(buf), fp)) {
 		int vifi;
 		char dev[256];