Reset segment_id to 0 when segmentation is disabled

Whilst the encoder explicitly set the segment_id to 0
when segmentation is diabled, the decoder would allow
the segment_id to persist from the previous frame.

This fix attempts to make the decoder behave the same
as the encoder by explicitly setting the segment_id to
0 in this case.

Change-Id: I65c3a05247550edb10706eb5d54d306dfb792309