gpg: Fix regression in --locate-keys (in 2.1.9).

* g10/getkey.c (getkey_ctx_s): Add field "extra_list".
(get_pubkey_byname): Store strings in the context.
(getkey_end): Free EXTRA_LIST.

This fixes a use-after-free bug. It showed up with:

gpg --auto-key-locate local --locate-key wk@gnupg.org

The key was shown but also all other following keys in the keyring.
Bisecting showed d47e84946ee010917cfc3501062721b74afbb771 as culprit
but the actual cause was a part of:

• Regression-due-to: b06f96ba4f57f55194efcd37a0e3a2aa5450b974
• Signed-off-by: Werner Koch <wk@gnupg.org>