*: add/cleanup initialisers

There were some (inconsequential) warnings about uninitialised use of
variables.  Also, in one case, sub-structs were mixed in initialisation,
which doesn't quite work as intended.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/ripngd/ripngd.c b/ripngd/ripngd.c
index 8c20a7a..82e3818 100644
--- a/ripngd/ripngd.c
+++ b/ripngd/ripngd.c
@@ -236,7 +236,7 @@
   struct msghdr msg;
   struct iovec iov;
   struct cmsghdr  *cmsgptr;
-  struct in6_addr dst;
+  struct in6_addr dst = { .s6_addr = { 0 } };
 
   /* Ancillary data.  This store cmsghdr and in6_pktinfo.  But at this
      point I can't determine size of cmsghdr */