crypto: Fix certificate file error handling crash bug

crypto: Fix certificate file error handling crash bug

qcrypto_tls_creds_load_cert() passes uninitialized GError *gerr by
reference to g_file_get_contents().  When g_file_get_contents() fails,
it'll try to set a GError.  Unless @gerr is null by dumb luck, this
logs a ERROR_OVERWRITTEN_WARNING warning message and leaves @gerr
unchanged.  qcrypto_tls_creds_load_cert() then dereferences the
uninitialized @gerr.

Fix by initializing @gerr properly.

Fixes: 9a2fd4347c40321f5cbb4ab4220e759fcbf87d03
Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191204093625.14836-2-armbru@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>