mshtml: Make IHTMLElement::get_document vtbl offset independent.

dlls/mshtml/htmlelem.c

@@ -585,7 +585,7 @@ static HRESULT WINAPI HTMLElement_get_document(IHTMLElement *iface, IDispatch **
     if(!p)
         return E_POINTER;
 
-    *p = (IDispatch*)This->node.doc;
+    *p = (IDispatch*)HTMLDOC(This->node.doc);
     IDispatch_AddRef(*p);
 
     return S_OK;

dlls/mshtml/tests/dom.c

@@ -3343,6 +3343,11 @@ static void test_elems(IHTMLDocument2 *doc)
         ok(type == 1, "type=%ld\n", type);
 
         IHTMLSelectElement_Release(select);
+
+        hres = IHTMLElement_get_document(elem, &disp);
+        ok(hres == S_OK, "get_document failed: %08x\n", hres);
+        ok(iface_cmp((IUnknown*)disp, (IUnknown*)doc), "disp != doc\n");
+
         IHTMLElement_Release(elem);
     }