Fix to possible crash in R_IsViewpointThirdPerson

Basically to explain, due to R_InterpolateView being called in ST_Drawer it can possibly make the game pass an invalid player ref to R_IsViewpointThirdPerson during the first few tics of a level. This can happen because the player mobj hasn't been created yet so the view remains uninitialized. This seems to mostly affect legacyGL shearing as far as I can tell since it calls this function.

Thanks to GenericHeroGuy for helping me figure this out.