Reroute submix HAL: fix race condition on output state

When reading from a pipe, the output may have been previously
 closed, therefore the output state should only be read if it
 is still available.
This fixes a race condition observed between in_read() (which
 accesses the output stream) and adev_close_output_stream()
 (which sets the output reference to NULL).
 No issue with out_write() which checks the input reference.

Bug 16009464

Change-Id: I979bc12c8fe91fad9b6f6c9e0be107c1bacae360
1 file changed