Pull up following revision(s) (requested by spz in ticket #813):

sys/net/if_vlan.c: revision 1.122

If cnt == 0, don't kmem_alloc(0). Found by Mootja.

Looking at the code, I also find it suspicious that we read
ifv->ifv_mib->ifvm_p directly without making sure ifv_mib != NULL.

(martin)