Merged revisions 174710 via svnmerge from

https://origsvn.digium.com/svn/asterisk/trunk

........
  r174710 | file | 2009-02-10 16:15:43 -0400 (Tue, 10 Feb 2009) | 4 lines
  
  Only decrease inringing count if above zero.
  (issue #13238)
  Reported by: kowalma
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git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.0@174711 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -4643,7 +4643,9 @@ static int update_call_counter(struct sip_pvt *fup, int event)
 			sip_pvt_lock(fup);
 			ast_mutex_lock(pu_lock);
 			if (ast_test_flag(&fup->flags[0], SIP_INC_RINGING)) {
-				(*inringing)--;
+				if (*inringing > 0) {
+					(*inringing)--;
+				}
 				ast_clear_flag(&fup->flags[0], SIP_INC_RINGING);
 			}
 			ast_mutex_unlock(pu_lock);