playbin: Keep inputselector around until we release its pads

playbin: Keep inputselector around until we release its pads

Otherwise there's an interesting race condition when we destroy
the inputselector (actually it will be destroyed later when its state
change message gets destroyed) and afterwards release its sinkpad.

This is the code path when the last channel is removed from the
input selector.

Gave this warning sometimes, for chained oggs or whenever else
we change decode groups:
GStreamer-CRITICAL **: Padname '':sink_0 does not belong to element inputselector0 when removing