Teleport.cs: Check for existence of controllers before accessing them

[docquantum]

When trying to work on a project without an HMD with controllers, the project defaults to 2D Debug mode. However, since the code expects the controllers to exist, it's not able to get past trying to execute haptic feedback which results in teleporting to fail in 2D Debug mode. These changes make sure that the script check for controller existence before trying to access them.

This fixes issue #483

[keithbradner]

Thanks for the pull request from 2 years ago! This will go into the next release.