libasignify: streamline asignify_verify_load_signature
libasignify: streamline asignify_verify_load_signature

This small re-work makes it a little easier to follow the sucess-path
of signature verification, and fixes a memory leak of `data` if
asignify_verify_load_sig had a file to work with but we had already hit
EOF.

Signed-off-by: Kyle Evans <bdbb8922150f6bdb47902fe294c5e84994ac55b2@FreeBSD.org>

vstakhov/asignify,vstakhov/asignify