Rectify bug in lossless incremental decoding.

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).

Change-Id: Ibac9fcfc4bd0a2c5f624bb9d4a2b9f6459aa19ea