ubusd: handle invoke on event object without data

When a built-in object is invoked with UBUS_ATTR_DATA absent, recv_msg
will be called with NULL as the msg argument and ubusd_forward_event
and ubusd_alloc_event_pattern need to handle this. Otherwise, a
truncated invoke of "send" or "register" on UBUS_SYSTEM_OBJECT_EVENT
that is missing UBUS_ATTR_DATA will cause ubusd to crash with SIGSEGV.

Signed-off-by: Erik Karlsson <erik.karlsson@genexis.eu>

ubusd_event.c

@@ -63,6 +63,9 @@ static int ubusd_alloc_event_pattern(struct ubus_client *cl, struct blob_attr *m
 	bool partial = false;
 	int len;
 
+	if (!msg)
+		return UBUS_STATUS_INVALID_ARGUMENT;
+
 	blobmsg_parse(evr_policy, EVREG_LAST, attr, blob_data(msg), blob_len(msg));
 	if (!attr[EVREG_OBJECT] || !attr[EVREG_PATTERN])
 		return UBUS_STATUS_INVALID_ARGUMENT;
@@ -209,6 +212,9 @@ static int ubusd_forward_event(struct ubus_client *cl, struct blob_attr *msg)
 	struct blob_attr *attr[EVMSG_LAST];
 	const char *id;
 
+	if (!msg)
+		return UBUS_STATUS_INVALID_ARGUMENT;
+
 	blobmsg_parse(ev_policy, EVMSG_LAST, attr, blob_data(msg), blob_len(msg));
 	if (!attr[EVMSG_ID] || !attr[EVMSG_DATA])
 		return UBUS_STATUS_INVALID_ARGUMENT;