cpp: Initialize all gpgme_key_t's in context
cpp: Initialize all gpgme_key_t's in context

* lang/cpp/src/context.cpp (Context::startKeyListing),
(Context::keyListResult, Context::signingKeys): Initialize key.

--
"Should" not be neccessary but it's cleaner and would have
covered the case of the previous commit.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme