It's possible for a hangup to be received, even just after the initial cid

spill.
(closes issue #12453)
 Reported by: Alex728
 Patches: 
       20080604__bug12453.diff.txt uploaded by Corydon76 (license 14)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@124182 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_dahdi.c

@@ -6252,7 +6252,11 @@ static void *ss_thread(void *data)
 							ast_hangup(chan);
 							return NULL;
 						} 
-						f = ast_read(chan);
+						if (!(f = ast_read(chan))) {
+							ast_log(LOG_WARNING, "Hangup received waiting for ring. Exiting simple switch\n");
+							ast_hangup(chan);
+							return NULL;
+						}
 						ast_frfree(f);
 						if (chan->_state == AST_STATE_RING ||
 						    chan->_state == AST_STATE_RINGING)