Only waste 1 byte in queue buffer
Only waste 1 byte in queue buffer

For simplicity of the code, we don't allow (head == tail) when the queue
is full. But currently we are wasting the size of a single unit, while
we can actually just waste 1 byte. Let's fix this.

BUG=None
TEST=Pass the unit test.
BRANCH=None

Change-Id: Id09c20a9345ebb3ec0cad659afe36e25b422e688
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/58058
Reviewed-by: Yung-Chieh Lo <2d80bee8408e64f859a1b5868bd3b311c0a4b4cd@chromium.org>

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