Handle the corner case when VP8LDecodeImage() method is called with an invalid
header data. The lossless decoding doesn't support incremental mode yet.
Return the error status as BITSTREAM error in case not all pixels are decoded
with the provided bit-stream. Also added asserts in the VP8LDecodeImage() method
to validate the decoder header with appropriate/valid data for huffman trees
(htree_groups_ etc).
(cherry picked from commit e0a99321611c5b5a43e33b4d377877d999e730f9)
Change-Id: Ibac9fcfc4bd0a2c5f624bb9d4a2b9f6459aa19ea