core: Fix segv in genkey when no endtag is provided.
* src/genkey.c (get_key_parameter): Provide a fallback ENDTAG.
It would actually be more correct to return an error in this case but
it is possible tha there are users who did not provide an endtag and
out of luck they also didn't trigger a segv.
- GnuPG-bug-id: T4192
- Signed-off-by: Werner Koch <firstname.lastname@example.org>