A frame should be referenced with a unique delta,
which is not guaranteed when diff_len is greater than frame_id_length.

Otherwise, it's possible to get into a situation where a frame
can be referenced with many different delta_frame_id values,
although the decoder only accepts one of them.

Avoid ambiguities when delta-referencing a frame, and
avoid wasting bits by using a coding enforcing the restriction.

Change-Id: If7c611c9b6ec69662c5af540ade59f08bacda2ba