Reset icon ref count when deleting the icon in wxMSW balloon icons code.

This avoids the assert which happened when trying to show the next
notification message in SetUpIcon() as the icon was null while the reference
count was positive.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76447 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/notifmsg.cpp

@@ -341,6 +341,7 @@ wxBalloonNotifMsgImpl::DoShow(const wxString& title,
         // first place.
         delete ms_icon;
         ms_icon = NULL;
+        ms_refCountIcon = 0;
 
         return false;
     }