fix(SDK): prevent null exception if no controller model found

The SteamVR Controller SDK could potentially throw an exception
if no controller model was found because the `Transform.Find` method
could potentially find `null` but the `gameObject` getter was being
chained on the end of the `Find` call.

The fix is to split the line into two and do a null check before
attempting to get the gameObject.

Author: thestonefox

Assets/VRTK/SDK/SteamVR/SDK_SteamVRController.cs

@@ -333,7 +333,8 @@ public override GameObject GetControllerModel(ControllerHand hand)
 
                 if (controller != null)
                 {
-                    model = controller.transform.Find("Model").gameObject;
+                    Transform foundModel = controller.transform.Find("Model");
+                    model = (foundModel != null ? foundModel.gameObject : null);
                 }
             }
             return model;