Fix show_existing_frame not decreasing frame buffer ref counter.

The issue was introduced by commit g7c43fb6. If current frame
is repeated from existing-ref pool, frame buffer ref counter
is not decreased, so buffer isn't released. Decoder fails being
unable to allocate new frame buffer at some point.

Added a test vector to verify that the condition will not
recur later. Test vector was generated by the code in this patch:
https://gerrit.chromium.org/gerrit/#/c/70862/

Change-Id: I8af96eb5b9670176e01a281d2e18bd458712cf78