gpg: Extra check for sign usage when verifying a data signature.
* g10/sig-check.c (check_signature_end_simple): Check sign usage.
Without this patch the signature verification fails only due to the
missing back signature. This check better explains what went wrong.
(cherry picked from commit 214b0077264e35c079e854a8b6374704aea45cd5)
- GnuPG-bug-id: T4014
- Signed-off-by: Werner Koch <wk@gnupg.org>