The poll for unlinking calls nla_panel_context without providing an
adt pointer, and there is a check for this pointer in
nla_panel_context leading to never returning true if it is not
provided. (this is fine if there are tracks already, poll would succeed
in this case, nla_panel_context goes a different code path then)

Same call to nla_panel_context is also done in the beginning of the
corresponding unlink exec function (but this time providing the pointer
because it is used later), so it makes sense to do the same thing in the
poll function. Equal check is also done in the panel poll function, so
now these are all in sync.

Part of T87681 (Bug 2).