Unset metatable when destroying C++-objects.

When using Lua 5.2's __gc or luabinds __finalize metamethods, it is possible to
access userdata after its __gc metamethod has been called. Unsetting its
metatable turns undefined behavior in an ordinary Lua error (LUA_ERRGCMM).

Assuming CppClass is a C++-class exported by luabind, the following code
triggers undefined behavior (needs Lua 5.2 for __gc support for tables):
    t = { }
    setmetatable(t, {__gc = function(t)
        t.cppClass:method() end})
    t.cppClass = CppClass()

Since "the finalizers for objects are called in the reverse order that they
were marked for collection" [1], this will try to call the __index metamethod
of t.cppClass after the object_rep for it (and itself) has been destroyed. The
call to t.cppClass:method() results in undefined behavior (in the particular
case where I noticed the bug (on VS11, Win 7 x64) it was a pure virtual
function call to luabind::detail::instance_holder::get()). With this patch
applied, Lua will instead issue a runtime error because of indexing an userdata
without __index metamethod.

See also
    [1]: http://www.lua.org/manual/5.2/manual.html#2.5.1
    http://lua.2524044.n2.nabble.com/userdata-access-after-gc-td7643253.html

src/object_rep.cpp

@@ -91,6 +91,9 @@ namespace luabind { namespace detail
 
         instance->release_dependency_refs(L);
         instance->~object_rep();
+
+        lua_pushnil(L);
+        lua_setmetatable(L, 1);
         return 0;
     }