Implement error tracking in the decoder

A new vpx_codec_control called VP8D_GET_FRAME_CORRUPTED. The output
from the function is non-zero if the last decoded frame contains
corruption due to packet losses.

The decoder is also modified to accept encoded frames of zero length.
A zero length frame indicates to the decoder that one or more frames
have been completely lost. This will mark the last decoded reference
buffer as corrupted. The data pointer can be NULL if the length is
zero.

Change-Id: Ic5902c785a281c6e05329deea958554b7a6c75ce