RigCombMultResultAccessor::cellFaceScalar called oppositeFace() with the
face passed from the picked selection. A plain cell pick leaves the face
as NO_FACE, which has no opposite and tripped CVF_ASSERT(false) in
oppositeFace, crashing the application.
Guard against a non-directional face and return 1.0, consistent with the
no-change-in-MULT-factor convention already used in nativeMultScalar.