Fix to frames considered in arf boost calculation.

Fix to frames considered in arf boost calculation.

For a chosen interval "i" the existing arf boost calculation examined frames
+/- (i-1) frames from the current location in the second pass.

This change checks to make sure that the forward search does not extend
beyond the next key frame in the event that the distance to the next key
frame is < (i - 1).

Small metrics gains on all our  test sets but these are localized to a few clips
(e.g. midres set psnr-hvs sintel -2.59% but overall average was only -0.185%)

Change-Id: I26fc9ce582b6d58fa1113a238395e12ad3123cf6