b2b_entities: preserved leg index when answering call

Introduced in 862fa53, when the `pass-legs-upstream` flag was used and
the call was answered by any other leg but the first one, the index of
the leg has to be "preserved", otherwise the subsequent requests will
contain index 1, not the actual index.

(cherry picked from commit 73ef9dd)

Author: razvancrainea

modules/b2b_entities/dlg.c

@@ -3882,6 +3882,7 @@ void b2b_tm_cback(struct cell *t, b2b_table htable, struct tmcb_params *ps)
 				{
 					b2b_dlginfo_t dlginfo;
 					b2b_add_dlginfo_t add_infof= dlg->add_dlginfo;
+					int confirmed_leg_id = b2b_get_leg_index(dlg, &to_tag);
 
 					/* delete all and add the confirmed leg */
 					b2b_delete_legs(&dlg->legs);
@@ -3891,6 +3892,8 @@ void b2b_tm_cback(struct cell *t, b2b_table htable, struct tmcb_params *ps)
 						LM_ERR("Failed to add dialog leg\n");
 						goto error;
 					}
+					if (confirmed_leg_id >= 0)
+						leg->id = confirmed_leg_id;
 					dlg->tag[CALLEE_LEG] = leg->tag;
 					dlginfo.fromtag = to_tag;
 					dlginfo.callid = dlg->callid;