[tm] fix the visibility of $T_branch_idx in failure route

The bavp's are already available in failure route and they are using the same internal variable for the branch index. So, to keep consistency, the branch index should be visible also in failure route via $T_branch_idx

(cherry picked from commit ae9e51e)

modules/tm/doc/tm_admin.xml

@@ -1953,9 +1953,10 @@ route{
 			<para>
 			<emphasis>$T_branch_idx</emphasis> - the index (starting with 0
 			for the first branch) of the currently proccessed branch. This
-			index makes sense only in BRANCH and REPLY routes (where there is
-			a concept of per branch processing). In all the other types of 
-			routes, the value of this index will be NULL.
+			index makes sense only in BRANCH and REPLY routes (where the
+			processing is per branch) and in FAILURE route (where it points
+			to the branch with the last final reply on the transaction). In all
+			the other types of routes, the value of this index will be NULL.
 			</para>
 		</section>
 		<section id="pv_T_reply_code" xreflabel="$T_reply_code">

modules/tm/tm.c

@@ -1691,7 +1691,8 @@ static int pv_get_tm_branch_idx(struct sip_msg *msg, pv_param_t *param,
 	if(msg==NULL || res==NULL)
 		return -1;
 
-	if (route_type!=BRANCH_ROUTE && route_type!=ONREPLY_ROUTE) {
+	if (route_type!=BRANCH_ROUTE && route_type!=ONREPLY_ROUTE &&
+	route_type!=FAILURE_ROUTE) {
 		res->flags = PV_VAL_NULL;
 		return 0;
 	}