BGE: Fix for bug #34349 "Character walkDirection ADD mode -#INF error" reported by Angus Hollands (agoose77). If the walk directions canceled each other out, the actuator would try to normalize a zero vector, which caused the error.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54738 954f8c5b-7b00-dc11-b283-0030488c597c