make sure gonnaUse has a length

src/renderers/WebGLRenderer.js

@@ -3266,6 +3266,8 @@ THREE.WebGLRenderer = function ( parameters ) {
 				gonnaUse.length = material.numSupportedMorphTargets 
             } else if (gonnaUse.length >  material.numSupportedMorphNormals){
             	gonnaUse.sort(sortNumerical);
+            } else if (gonnaUse.length = 0){
+				gonnaUse.push(0);
             };
 
 			while ( m < material.numSupportedMorphTargets ) {