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