chan_sip: sip_subscribe_mwi_destroy should not call sip_destroy

sip_subscribe_mwi_destroy calls sip_destroy on the reference counted
mwi->call.  This results in the fields of mwi->call being freed, but
mwi->call itself it leaked.  If other code is still using mwi->call
it can cause problems.  This change uses dialog_unref instead, to
balance the ref provided by sip_alloc().

ASTERISK-24087 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3834/
........

Merged revisions 419440 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@419441 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -6450,7 +6450,7 @@ static void sip_subscribe_mwi_destroy(struct sip_subscription_mwi *mwi)
 {
 	if (mwi->call) {
 		mwi->call->mwi = NULL;
-		sip_destroy(mwi->call);
+		mwi->call = dialog_unref(mwi->call, "sip_subscription_mwi destruction");
 	}
 
 	AST_SCHED_DEL(sched, mwi->resub);