Fix for keyed marker search not happening correct in some cases

Was an heuristic failure writen for cases when there's no keyframe
in desired direction. Shall all be fine now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56726 954f8c5b-7b00-dc11-b283-0030488c597c