gpg: Fix possible segv when attribute packets are filtered.

* g10/import.c (impex_filter_getval): Handle PKT_ATTRIBUTE the same as
PKT_USER_ID
(apply_drop_sig_filter): Ditto.

The old code was plainly wrong in that it considered PKT_ATTRIBUTE to
use a PKT_signature object.

• Signed-off-by: Werner Koch <wk@gnupg.org>