iptables: close open file descriptors

Just for correctness, close some file descriptors that were opened.
(E.g. ip6tables-save reading from procfs files.)

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/iptables-restore.c b/iptables-restore.c
index 2a797cc..5108fda 100644
--- a/iptables-restore.c
+++ b/iptables-restore.c
@@ -459,5 +459,7 @@
 		exit(1);
 	}
 
+	if (in != NULL)
+		fclose(in);
 	return 0;
 }