Trap the case where we end up with a very short arf group just before
a key frame. Such a group often has poor quality and may cause pulsing.

For example if the KF is 17 frames away we are better doing two mid-size
groups of 9 and 8 than a group of 15 followed by a group of 2.

This becomes more and more important when coding with a short forced
kf interval though it may not impact our standard tests much.

Change-Id: I29d83d6637b203eac69be320dd35a7401a4678c1