fix(SDK): prevent crash when requesting non-existant vive render model
fix(SDK): prevent crash when requesting non-existant vive render model

The `GetControllerRenderModel` method in the SteamVR SDK can cause a
crash if the scene is not using the default vive controller render
model because the component cannot be retrieved yet the return
statement expects a game object to exist.

The fix is to just check that the found render model exists before
attempting to return the associated game object.

red-horizon/VRTK,pargee/SteamVR_Unity_Toolkit,thestonefox/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,adamjweaver/VRTK,nmccarty/htvr,TMaloteaux/SteamVR_Unity_Toolkit,gpvigano/SteamVR_Unity_Toolkit,thestonefox/VRTK,gpvigano/VRTK,gomez-addams/SteamVR_Unity_Toolkit,Fulby/VRTK,mattboy64/SteamVR_Unity_Toolkit,virror/VRTK