Initialize both luma and chroma planes in palette encoding
Initialize both luma and chroma planes in palette encoding

This avoids the use of uninitialized value in palette encoding.

BUG=aomedia:568

Change-Id: I1000b28d9acf25a70ef641d4494f0b116a4ca071

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom