pv: handle s.prefixes for empty values

- avoid crash due to invalid length of result, GH #2467

src/modules/pv/pv_trans.c

@@ -909,6 +909,7 @@ int tr_eval_string(struct sip_msg *msg, tr_param_t *tp, int subtype,
 			if(!(val->flags&PV_VAL_STR))
 				val->rs.s = int2str(val->ri, &val->rs.len);
 
+
 			/* Set maximum prefix length */
 			max = val->rs.len;
 			if(tp!=NULL) {
@@ -947,7 +948,7 @@ int tr_eval_string(struct sip_msg *msg, tr_param_t *tp, int subtype,
 			memset(val, 0, sizeof(pv_value_t));
 			val->flags = PV_VAL_STR;
 			val->rs.s = _tr_buffer;
-			val->rs.len = j-1;
+			val->rs.len = (j>0)?(j-1):0;
 			break;