gpg: Fix buglet in the check_all_keysigs function.

* g10/keyedit.c (sig_comparison): Actually compare the pubkey
algorithms.

This fixes two bugs: The first was a typo which led to us comparing A
with A. The second problem was the use of an assert at a place where
this can't be asserted: Two signature may have different algorithms;
they won't verify but after all it is about corrupted signatures.

• Reported-by: Guilhem Moulin <guilhem@fripost.org>
• GnuPG-bug-id: T2236
• Signed-off-by: Werner Koch <wk@gnupg.org>