vp10: don't reset contextual skip flag if block has no coefficients.

vp10: don't reset contextual skip flag if block has no coefficients.

The implicitly changed value would be used for contextualizing future
skip flags of neighbour blocks (bottom/right), which is certainly not
what was intended. The original code stems from vp8, and was useful
in cases where coding of the skip flag was disabled. In vp9, the skip
flag is always coded. The result of this change is that for bitstream
parsing purposes, decoding of the skip flag becomes independent of
decoding of block coefficients.

See issue 1014.

Change-Id: I8629e6abe76f7c1d649f28cd6fe22a675ce4a15d