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.

src/r_main.cpp

@@ -1430,7 +1430,13 @@ boolean R_ViewpointHasChasecam(player_t *player)
 
 boolean R_IsViewpointThirdPerson(player_t *player, boolean skybox)
 {
-	boolean chasecam = R_ViewpointHasChasecam(player);
+	boolean chasecam = false;
+
+	// Prevent game crash if player is ever invalid.
+	if (!player)
+		return false;
+		
+	chasecam = R_ViewpointHasChasecam(player);
 
 	// cut-away view stuff
 	if (player->awayview.tics || skybox)