pbrd: fix removal of ipv6 nexthops

pbrd: fix removal of ipv6 nexthops

Fix bug in the code that compares IPv6 addresses. If memcmp()
returns 0 then the two addresses are equal.

Because of this problem, hash_release() could return NULL in a few
places, leading to the following crashes (found by the CLI fuzzer):
pbrd aborted: vtysh -c "configure terminal" -c "pbr-map WORD seq 100" -c "no set nexthop 2001:db8::1"
pbrd aborted: vtysh -c "configure terminal" -c "nexthop-group NHGROUP" -c "no nexthop 2001:db8::1"

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>