vo: free frames before killing VO

This caused issues with hardware decoding. The VOs by definition dictate
the lifetime of the hardware context, so no surface allocations must
survive the VO. Fixes assertions on exit with vdpau.

video/out/vo.c

@@ -822,10 +822,10 @@ static void *vo_thread(void *ptr)
         }
         wait_vo(vo, wait_until);
     }
-    vo->driver->uninit(vo);
     forget_frames(vo); // implicitly synchronized
     talloc_free(in->current_frame);
     in->current_frame = NULL;
+    vo->driver->uninit(vo);
     return NULL;
 }