Fix a bug in jnt_comp

(1). index may go out side of range
(2). when d0 <= d1, comparison is invalid.

Performance impact on Google lowres testset:
Turn on jnt_comp vs baseline,
Without fix: -0.211% gain
With fix: -0.357% gain

BUG=aomedia:1239

Change-Id: I761522bba8396bba0d4108d710030b472939cf32