esound,streams: Fix some crashes.

esound,streams: Fix some crashes.

After the rework to the add pa_sink_input_new_data_set_sink() (and
the source equiv) calling with a NULL sink object will hit an assert.

This caused crashes with the esd protocol and there was the potential
(albeit unlikely) for a crash when creating a sink input without any
sinks available (module-always-sink mitigates this risk but it's still
a potential crasher).