appsrc/sink: Fix optimization for only signalling waiters if someone is actually...

appsrc/sink: Fix optimization for only signalling waiters if someone is actually waiting

It is possible that both application and the stream are waiting
currently, if for example the following happens:
  1) app is waiting because no buffer in appsink
  2) appsink providing a buffer and waking up app
  3) appsink getting another buffer and waiting because it's full now
  4) app thread getting back control

Previously step 4 would overwrite that the appsink is currently waiting,
so it would never be signalled again.

https://bugzilla.gnome.org/show_bug.cgi?id=795551