Initialize variables to avoid segfault.

If we call 'keystone_end' right after 'keystone_start'
segfault occurs.
It's clear that it's not expected user behavior,
but possible segfaults should be avoided.

keystone-client.c

@@ -169,6 +169,9 @@ keystone_start(keystone_context_t *context)
 		return KSERR_INIT_FAILED;
 	}
 
+	context->pvt.auth_token = NULL;
+	context->pvt.auth_payload = NULL;
+	context->pvt.json_tokeniser = NULL;
 	return KSERR_SUCCESS;
 }