Re-fix how local tag is generated when sending a 481 to an INVITE.

Match our local tag to whatever to-tag was sent in the initial INVITE.
Because the size of the to-tag may not fit in the buffer in the sip_pvt,
it has been changed to a string field.

(closes issue ASTERISK-19892)
reported by Walter Doekes

Review: https://reviewboard.asterisk.org/r/1977



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@369352 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/sip/include/sip.h

@@ -959,6 +959,7 @@ struct sip_pvt {
 		AST_STRING_FIELD(rdnis);        /*!< Referring DNIS */
 		AST_STRING_FIELD(redircause);   /*!< Referring cause */
 		AST_STRING_FIELD(theirtag);     /*!< Their tag */
+		AST_STRING_FIELD(tag);          /*!< Our tag for this session */
 		AST_STRING_FIELD(username);     /*!< [user] name */
 		AST_STRING_FIELD(peername);     /*!< [peer] name, not set if [user] */
 		AST_STRING_FIELD(authname);     /*!< Who we use for authentication */
@@ -1006,7 +1007,6 @@ struct sip_pvt {
 	                                       *   for incoming calls
 	                                       */
 	unsigned short req_secure_signaling:1;/*!< Whether we are required to have secure signaling or not */
-	char tag[11];                     /*!< Our tag for this session */
 	int timer_t1;                     /*!< SIP timer T1, ms rtt */
 	int timer_b;                      /*!< SIP timer B, ms */
 	unsigned int sipoptions;          /*!< Supported SIP options on the other end */