pulse: Bail early from pulse_defer_event_cb when shutting down.
pulse: Bail early from pulse_defer_event_cb when shutting down.

When starting a stream, trigger_user_callback may be called from
stream_write_callback and immediately enter a drain situation, creating
a drain timer and setting shutdown to true.  If pulse_defer_event_cb
then runs without checking for shutdown, it can overwrite the current
timer with a new timer, resulting in a leaked timer and a null pointer
assertion.

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