Skip the last keyframe operation of KeyframeTrackPrototype.optimize() if times.length <= 1 (#9691)

src/animation/KeyframeTrackPrototype.js

@@ -336,6 +336,8 @@ KeyframeTrackPrototype = {
 
 		// flush last keyframe (compaction looks ahead)
 
+		if ( lastIndex > 0 ) {
+
 			times[ writeIndex ] = times[ lastIndex ];
 
 			for ( var readOffset = lastIndex * stride, writeOffset = writeIndex * stride, j = 0; j !== stride; ++ j )
@@ -344,6 +346,8 @@ KeyframeTrackPrototype = {
 
 			++ writeIndex;
 
+		}
+
 		if ( writeIndex !== times.length ) {
 
 			this.times = AnimationUtils.arraySlice( times, 0, writeIndex );