Nullptr dereference in FHeadMountedDisplay::UpdateSplashScreen()

A customer has had problems running a game with a splash screen when passing -game flag to the editor. UpdateSpashScreen will call GetCurrentFrame, which can return a nullptr but fails to verify it before using the pointer.

The fix may be simply avoiding the nullptr reference by checing the result, but we should possibly check why GetCurrentFrame is returning null at this point.

1. Plug in an oculus headset (make sure other HMDs are disconnected)
2. Open the TM-VRSmoke map in QAGame
3. To the level Blueprint, on BegingPlay, add a SetSplashScreen node from the StereoLayerFunctionLibrary
4. Set the node's texture param (I selected the "512" rainbow texture)
5. Check the node's ShowOnSet param
6. Save the level Blueprint and close the editor
7. Relaunch the Editor (directly into that level) with -game

RESULT: crash in FOculusHMD::UpdateSplashScreen()