fix 349504 error adding rss feed through subscribe dialog, sr=mscott

mail/extensions/newsblog/content/FeedItem.js

@@ -419,11 +419,11 @@ FeedItem.prototype =
 
     // Get the folder and database storing the feed's messages and headers.
     folder = this.feed.folder.QueryInterface(Components.interfaces.nsIMsgLocalMailFolder);
-
-    this.feed.folder.gettingNewMessages = true;
+    var msgFolder = folder.QueryInterface(Components.interfaces.nsIMsgFolder);
+    msgFolder.gettingNewMessages = true;
     // source is a unicode string, we want to save a char * string in the original charset. So convert back
     folder.addMessage(this.mUnicodeConverter.ConvertFromUnicode(source));
-    this.feed.folder.gettingNewMessages = false;
+    msgFolder.gettingNewMessages = false;
     this.markStored();
   }
 };