gpg: Fix NULL de-ref for ambiguous key check in --export-ssh-keys.
* g10/getkey.c: Allow arg RET_KEYBLOCK to be NULL.
This change adds the expected behavior for the getkey_next function
to fix this NULL de-ref.
- GnuPG-bug-id: T2212
- Signed-off-by: Werner Koch <wk@gnupg.org>