ubusd: handle invoke on event object without data

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>