libvpx

Fix AQ mode 2 bug where delta causes Q 0.

In Aq mode 2 for kf/arf/gf the segment q delta
is calculated and then applied by re-quantization without
going through the rd loop again. If the base Q != 0
but the segment Q == 0 (lossless) this can could give rise
to a situation where we have an illegal combination of
transform size and Q. (Q == 0 requires that all blocks
are coded 4x4 WHT).

Change-Id: I241a58c6494ed442e9e4630070b0cde0fb99ae45