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" <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20191204093625[email protected]>
Acked-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>