Fixed crash in stereoscopic modes caused by camera without player

https://forum.zdoom.org/viewtopic.php?t=55039&start=381#p1039251

src/gl/scene/gl_scene.cpp

@@ -842,7 +842,7 @@ sector_t * GLSceneDrawer::RenderViewpoint (AActor * camera, GL_IRECT * bounds, f
 	stereo3dMode.SetUp();
 	for (int eye_ix = 0; eye_ix < stereo3dMode.eye_count(); ++eye_ix)
 	{
-		if (eye_ix > 0)
+		if (eye_ix > 0 && camera->player)
 			SetFixedColormap(camera->player); // reiterate color map for each eye, so night vision goggles work in both eyes
 		const s3d::EyePose * eye = stereo3dMode.getEyePose(eye_ix);
 		eye->SetUp();