Fix a crash in map_is_known

When invoking map_is_known before the map is initialized, it would crash with
an assertion error in QVector.  This was the case for instance when calling it
indirectly from the "map_generated" callback (using tile:remove_extra()).

server/maphand.cpp

@@ -902,7 +902,8 @@ void map_show_all(struct player *pplayer)
  */
 bool map_is_known(const struct tile *ptile, const struct player *pplayer)
 {
-  return pplayer->tile_known->at(tile_index(ptile));
+  return !pplayer->tile_known->isEmpty()
+         && pplayer->tile_known->at(tile_index(ptile));
 }
 
 /**