Improve code readability

This also fixes https://lgtm.com/projects/g/mrdoob/three.js/snapshot/c47fb709c7f969c42bb0b8bb2316a8cf8a53754c/files/examples/js/loaders/AssimpLoader.js?sort=name&dir=ASC&mode=heatmap&excluded=false#x3eeae27c070bf750:1

examples/js/loaders/AssimpLoader.js

@@ -1128,9 +1128,11 @@ THREE.AssimpLoader.prototype = {
 
 			}
 
-			if ( ! key ) return null;
+			if ( ! key ) {
+				
+				return null;
 
-			if ( key && nextKey ) {
+			} else if ( nextKey ) {
 
 				var dT = nextKey.mTime - key.mTime;
 				var T = key.mTime - time;
@@ -1138,16 +1140,18 @@ THREE.AssimpLoader.prototype = {
 
 				return lerp( key.mValue.toTHREE(), nextKey.mValue.toTHREE(), l );
 
-			}
+			} else {
 
-			nextKey = keys[ 0 ].clone();
-			nextKey.mTime += lne;
+				nextKey = keys[ 0 ].clone();
+				nextKey.mTime += lne;
 
-			var dT = nextKey.mTime - key.mTime;
-			var T = key.mTime - time;
-			var l = T / dT;
+				var dT = nextKey.mTime - key.mTime;
+				var T = key.mTime - time;
+				var l = T / dT;
 
-			return lerp( key.mValue.toTHREE(), nextKey.mValue.toTHREE(), l );
+				return lerp( key.mValue.toTHREE(), nextKey.mValue.toTHREE(), l );
+				
+			}
 
 		}