in the great unp_gc rewrite, a null check was lost. we have spent some

in the great unp_gc rewrite, a null check was lost. we have spent some
time investigating and arguing about whether a NULL fp is a bug or not,
but what has become clear is that NULL fps get passed to unp_discard
and have been for quite some time.
restore old accomodating behavior by checking for null in unp_gc.
ok deraadt kettenis