ieframe: Don't crash if output parameter is NULL in WebBrowser::get_RegisterAsDropTarget.

dlls/ieframe/tests/webbrowser.c

@@ -2321,6 +2321,10 @@ static void test_ie_funcs(IWebBrowser2 *wb)
         ok(sName!=NULL, "get_Name return a NULL string.\n");
     SysFreeString(sName);
 
+    /* RegisterAsDropTarget */
+    hres = IWebBrowser2_get_RegisterAsDropTarget(wb, NULL);
+    ok(hres == E_INVALIDARG, "get_RegisterAsDropTarget returned: %08x\n", hres);
+
     /* Quit */
 
     hres = IWebBrowser2_Quit(wb);

dlls/ieframe/webbrowser.c

@@ -943,7 +943,12 @@ static HRESULT WINAPI WebBrowser_get_RegisterAsDropTarget(IWebBrowser2 *iface,
         VARIANT_BOOL *pbRegister)
 {
     WebBrowser *This = impl_from_IWebBrowser2(iface);
+
     FIXME("(%p)->(%p)\n", This, pbRegister);
+
+    if(!pbRegister)
+        return E_INVALIDARG;
+
     *pbRegister=0;
     return S_OK;
 }