*: fix more initialisers (for BSD)

FreeBSD and NetBSD spew a few more warnings about variable initialisers.
Found with OSR's/NetDEF's fancy new CI system.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/ripngd/ripngd.c b/ripngd/ripngd.c
index 82e3818..4c0b67d 100644
--- a/ripngd/ripngd.c
+++ b/ripngd/ripngd.c
@@ -1317,7 +1317,7 @@
   int sock;
   struct sockaddr_in6 from;
   struct ripng_packet *packet;
-  unsigned int ifindex;
+  unsigned int ifindex = 0;
   struct interface *ifp;
   int hoplimit = -1;