uORB: fix node_open: *instance is read even though it's an output parameter

This fixes a subtle bug: the instance parameter of orb_advertise is an output
parameter and thus its value can be random. However in node_open this value
is accessed and thus the open(...) call could succeed even though it should
not. This can happen for example if a second advertiser of a topic calls
orb_advertise_multi with *instance=0.

The existing implementation worked only because *instance was initialized
with -1 in most cases.