Fix possible use-after-free on error handling during VR view drawing

Whenever an exception happens in VR session code, we cancel the entire
session. Alongside that, we removed the "surface" item used to draw into
an offscreen context. But this may still be stored as active surface,
leading to a use-after-free when deactivating this active surface, for
example.

source/blender/windowmanager/intern/wm_surface.c

@@ -119,6 +119,9 @@ void wm_surface_add(wmSurface *surface)
 
 void wm_surface_remove(wmSurface *surface)
 {
+  if (surface == g_drawable) {
+    wm_surface_clear_drawable();
+  }
   BLI_remlink(&global_surface_list, surface);
   surface->free_data(surface);
   MEM_freeN(surface);