lib: unset ZEBRA_IFA_PEER if no dst addr present (BZ#801)

lib: unset ZEBRA_IFA_PEER if no dst addr present (BZ#801)

On OpenBSD, carp interfaces claim to be PtP interfaces with a 0.0.0.0/0
peer address.  We process those in zebra and try to send them to
clients, at which point they get encoded as all-0.  The client code,
however, decodes that to a NULL pointer instead of 0.0.0.0.  This later
turns into a SEGV when CONNECTED_PREFIX sees that ZEBRA_IFA_PEER is set
and tries to access the peer prefix.

This is a band-aid fix for stable/0.99.23, a long-term solution needs
some conceptual improvements on the entire thing.

(The usefulness of a PtP-to-0.0.0.0/0 is a separate question;  at this
point dropping the peer prefix seems the least intrusive solution.)

Reported-by: Laurent Lavaud <laurent.lavaud@ladtech.fr>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>