If there is no session timer in the INVITE, set it to default value (not unset minimum = -1)

Patch by oej

closes issue #15621
Reported by: fnordian
Tested by: atis


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@216695 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -20631,7 +20631,8 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 					st_interval = uac_max_se;
 				}
 			} else {
-				st_interval = uac_min_se;
+				/* Set to default max value */
+				st_interval = global_max_se;
 			}
 			break;