Allow assigning NULL lightuserdata to pointers, and export a global.

Otherwise there is no way to specify NULL via recursive lua table assign.

library/LuaTypes.cpp

@@ -215,12 +215,19 @@ static void autovivify_ptr(lua_State *state, int fname_idx, void **pptr,
     lua_pop(state, 1);
 }
 
+static bool is_null(lua_State *state, int val_index)
+{
+    return lua_isnil(state, val_index) ||
+           (lua_islightuserdata(state, val_index) &&
+            !lua_touserdata(state, val_index));
+}
+
 void df::pointer_identity::lua_write(lua_State *state, int fname_idx, void *ptr,
                                      type_identity *target, int val_index)
 {
     auto pptr = (void**)ptr;
 
-    if (lua_isnil(state, val_index))
+    if (is_null(state, val_index))
         *pptr = NULL;
     else if (lua_istable(state, val_index))
     {

library/LuaWrapper.cpp

@@ -496,14 +496,7 @@ static int meta_sizeof(lua_State *state)
         luaL_error(state, "Usage: object:sizeof() or df.sizeof(object)");
 
     // Two special cases: nil and lightuserdata for NULL and void*
-    if (lua_isnil(state, 1))
+    if (lua_isnil(state, 1) || lua_islightuserdata(state, 1))
-    {
-        lua_pushnil(state);
-        lua_pushinteger(state, 0);
-        return 2;
-    }
-
-    if (lua_islightuserdata(state, 1))
     {
         lua_pushnil(state);
         lua_pushnumber(state, (size_t)lua_touserdata(state, 1));
@@ -1314,6 +1307,11 @@ static void DoAttach(lua_State *state)
         lua_getfield(state, LUA_REGISTRYINDEX, DFHACK_ASSIGN_NAME);
         lua_setfield(state, -2, "assign");
 
+        lua_pushlightuserdata(state, NULL);
+        lua_setfield(state, -2, "NULL");
+        lua_pushlightuserdata(state, NULL);
+        lua_setglobal(state, "NULL");
+
         freeze_table(state, true, "df");
         lua_remove(state, -2);
         lua_setmetatable(state, -2);