ui/sdl2: fix surface_gl_update_texture: Assertion 'gls' failed

Before sdl2_gl_update() is called, sdl2_gl_switch() may decide to
destroy the console window and its associated shaders.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1644
Fixes: c84ab0a500a8 ("ui/console: optionally update after gfx switch")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20230511074217.4171842-1-marcandre.lureau@redhat.com>

ui/sdl2-gl.c

@@ -67,6 +67,10 @@ void sdl2_gl_update(DisplayChangeListener *dcl,
 
     assert(scon->opengl);
 
+    if (!scon->real_window) {
+        return;
+    }
+
     SDL_GL_MakeCurrent(scon->real_window, scon->winctx);
     surface_gl_update_texture(scon->gls, scon->surface, x, y, w, h);
     scon->updates++;