Fix bug 123768 - Unsubscribing while headers downloading hangs.

r=hwaara/sr=mscott.

mailnews/news/src/nsNNTPProtocol.cpp

@@ -3624,7 +3624,7 @@ PRInt32 nsNNTPProtocol::ReadXover(nsIInputStream * inputStream, PRUint32 length)
 
     m_numArticlesLoaded++;
     PR_FREEIF(line);
-    return status; /* keep going */
+    return NS_SUCCEEDED(rv) ? status : -1; /* keep going if no error */
 }