agent: Fix a leak in nice_output_stream_write() with a NULL cancellable

agent: Fix a leak in nice_output_stream_write() with a NULL cancellable

If @cancellable is NULL in a call to nice_output_stream_write(), the
WriteData struct is created with a reference count of 4, but only two
operations are scheduled which will result in its reference count being
decremented. The third operation is only scheduled if @cancellable is
non-NULL (and the final reference is dropped unconditionally at the end
of the function).

Fix this by properly implementing reference counting for WriteData,
rather than hard-coding the expected number of references in a fragile
and unmaintainable way.