pv: clone string in all substr operations, related to GH #1937

(cherry picked from commit 280eabf16a45f522655f8b49677e46c75d34024f)

src/modules/pv/pv_trans.c

@@ -614,6 +614,7 @@ int tr_eval_string(struct sip_msg *msg, tr_param_t *tp, int subtype,
 				}
 				val->rs.s += i;
 				val->rs.len = j;
+				tr_string_clone_result;
 				break;
 			}
 			i = -i;