Fix uninitialised bits after uncompressed header
Fix uninitialised bits after uncompressed header

A few bits of data were not zeroed because aom_wb_overwrite_literal
was being used instead of aom_wb_write_literal.

BUG=aomedia:1041

Change-Id: If9196551e99b6e5eeaefc3fe022088088f1dcd51

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