opt_dest should be assigned to not pushed

ol.geom.flat.interpolate.lineString will push result into opt_dest which only works if supplied array is empty. Because of this ol.geom.LineString.prototype.getFlatMidpoint will produce invalid results on geometries when they are modified. This pull request resolves the problem using a fix from @tschaub. I've also supplied a test case that requires the fix to pass.