webmdec: Fix read_frame return value for calls after EOS

webm_read_frame assumes that it won't be called once end of file
is reached. But for frame parallel mode that turns out to be not
true. this patch fixes that behavior by checking for EOS and
returning the appropriate value for subsequent calls.

Change-Id: Ie2fddbe00493a0f96c4172c67be1eb719f0fe8ed