core: Fix setting of the chain_model signature result.

* src/verify.c (parse_trust): Fix detection of "chain" keyword.

from 2007 shortly after introducing this. I doubt that this info has
ever been used (it is for qualified signatures, which are only
supported using the legacy German RegTP rules for them which were soon
overturned by the commercial CAs).

• Fixes-commit: da6f3dc0c53f11f7432b3c258702db07d6c37f9a
• Signed-off-by: Werner Koch <wk@gnupg.org>