Fixed handling of regularly placed keyframes

The encoder was not handling requests to place keyframes at
fixed intervals, i.e. kf_min_dist == kf_max_dist, correctly.

In this case when looking to place the next keyframe it was
accumulating stats all the way up to the end of the firstpass
file. This patch corrects this behavior.

Change-Id: I948ad9f1d7faa0c05861df588136cce3bb61d7e7