vp9-dynamic resize: Fix bug on releasing scaled reference.

vp9-dynamic resize: Fix bug on releasing scaled reference.

When the codec frame size is the same as the reference frame size,
release the scaled reference before assigning it a new buf_idx.
Only affects 1 pass non-svc mode, where the scaled references are
release only under certain conditions (to prevent un-needed scaling
of the references every frame).

Modified a unittest that can trigger this bug without this change.

https://code.google.com/p/chromium/issues/detail?id=582598

Change-Id: I9a884e36ebd7608b1641ec2a469e20a4f829cf43