Fix weak_ref bug.

See http://lua.2524044.n2.nabble.com/weak-ref-issue-patch-td7581558.html for an
explanation and a patch for an older luabind version, which I have just adapted.

Quote:
> This prevents the same id from being used by two weak_refs at the same time,
> which previously could result in the Lua portion of a wrap_base derived class
> from being lost or aliased by another object.

src/weak_ref.cpp

@@ -36,6 +36,7 @@ namespace
 {
 
   int weak_table_tag;
+  int impl_table_tag;
 
 } // namespace unnamed
 
@@ -59,7 +60,28 @@ LUABIND_API void get_weak_table(lua_State* L)
         lua_pushlightuserdata(L, &weak_table_tag);
         lua_pushvalue(L, -2);
         lua_rawset(L, LUA_REGISTRYINDEX);
+
+    }
+
+}
+
+LUABIND_API void get_impl_table(lua_State* L)
+{
+
+    lua_pushlightuserdata(L, &impl_table_tag);
+    lua_rawget(L, LUA_REGISTRYINDEX);
+
+    if (lua_isnil(L, -1))
+    {
+        lua_pop(L, 1);
+
+        lua_newtable(L);
+        lua_pushlightuserdata(L, &impl_table_tag);
+        lua_pushvalue(L, -2);
+        lua_rawset(L, LUA_REGISTRYINDEX);
+
     }
+
 }
 
 } // namespace luabind
@@ -74,15 +96,22 @@ namespace luabind
             , state(main)
             , ref(0)
         {
+
+            get_impl_table(s);
+            lua_pushlightuserdata(s, this);
+            ref = luaL_ref(s, -2);
+            lua_pop(s, 1);
+
             get_weak_table(s);
             lua_pushvalue(s, index);
-            ref = luaL_ref(s, -2);
+            lua_rawseti(s, -2, ref);
             lua_pop(s, 1);
+
         }
 
         ~impl()
         {
-            get_weak_table(state);
+            get_impl_table(state);
             luaL_unref(state, -1, ref);
             lua_pop(state, 1);
         }

src/wrapper_base.cpp

@@ -34,6 +34,7 @@ namespace luabind { namespace detail
 	LUABIND_API void do_call_member_selection(lua_State* L, char const* name)
 	{
 		object_rep* obj = static_cast<object_rep*>(lua_touserdata(L, -1));
+		assert(obj);
 		lua_pop(L, 1); // pop self
 
 		obj->crep()->get_table(L); // push the crep table