siprec: Fix invalid READ operation on freed memory

Also tidy up the cleanup phase

Thanks to Rob Moore, Daniel Bryars and Steven Ayre from Dubber for
reporting it and providing valuable information for troubleshooting!

(cherry picked from commit 9996f9f)

modules/siprec/siprec_logic.c

@@ -646,14 +646,19 @@ void srec_logic_destroy(struct src_sess *sess)
 {
 	if (!sess->b2b_key.s)
 		return;
-	shm_free(sess->b2b_key.s);
 
-	if (sess->initial_sdp.s)
+	if (sess->initial_sdp.s) {
 		shm_free(sess->initial_sdp.s);
+		sess->initial_sdp.s = NULL;
+	}
 
 	srec_b2b.entity_delete(B2B_CLIENT, &sess->b2b_key, sess->dlginfo, 1, 1);
-	if (sess->dlginfo)
+	if (sess->dlginfo) {
 		shm_free(sess->dlginfo);
+		sess->dlginfo = NULL;
+	}
+
+	shm_free(sess->b2b_key.s);
 	sess->b2b_key.s = NULL;
 
 	sess->flags &= ~SIPREC_STARTED;