textops: reinitialize new tr_param_t object for tr_txt_parse_re()

- avoid segfault when running fparam_free_restore() on
  such an object immediately after creation, because its field "next"
  contains garbage

(cherry picked from commit 38b8fbdf940b273cf541678fd22af7dd4d63ab3d)

src/modules/textops/txt_var.c

@@ -215,6 +215,7 @@ char* tr_txt_parse_re(str *in, trans_t *t)
 				LM_ERR("no more private memory!\n");
 				goto error;
 			}
+			memset(tp, 0, sizeof(tr_param_t));
 
 			se=subst_parser(&tok);