Account for changing morph target count (#20845)

src/renderers/webgl/WebGLMorphtargets.js

@@ -34,7 +34,7 @@ function WebGLMorphtargets( gl ) {
 
 		let influences = influencesList[ geometry.id ];
 
-		if ( influences === undefined ) {
+		if ( influences === undefined || influences.length !== length ) {
 
 			// initialise list