CRAS: empty_iodev - Set buffer to NULL after freeing.
CRAS: empty_iodev - Set buffer to NULL after freeing.

This avoid a double free if the device is closed twice.

BUG=none
TEST=cras_test_client --playback_file and let play to end, no crash.

Change-Id: Ib8c26571ab65ba44ece1baf502ff0621db8dc026
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/171648
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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