gcancellable: Assert that make_pollfd() call succeeds
The make_pollfd()
call can’t fail because it only does so if
cancellable == NULL
, and we’ve already checked that. Assert that’s the
case, to shut Coverity up and to catch behavioural changes in future.
Coverity CID: #1159433 Signed-off-by: Philip Withnall withnall@endlessm.com