fix horrible cast in find folders with flag

mailnews/base/util/nsMsgFolder.cpp

@@ -912,9 +912,11 @@ NS_IMETHODIMP nsMsgFolder::GetFoldersWithFlag(PRUint32 flags, nsIMsgFolder **res
 	PRUint32 cnt;
   rv = mSubFolders->Count(&cnt);
   if (NS_SUCCEEDED(rv)) {
-    for (PRUint32 i=0; i < cnt; i++) {
-      folder = getter_AddRefs((nsIMsgFolder*)mSubFolders->ElementAt(i));
-      if(NS_SUCCEEDED(rv))
+    for (PRUint32 i=0; i < cnt; i++) 
+	{
+		nsCOMPtr<nsISupports> supports = getter_AddRefs(mSubFolders->ElementAt(i));
+		folder = do_QueryInterface(supports, &rv);
+      if(NS_SUCCEEDED(rv) && folder)
       {
         // CAREFUL! if NULL is passed in for result then the caller
         // still wants the full count!  Otherwise, the result should be at most the