[master] 4ac5e49 don't panic for ws overflow in VBF_Get_Filter_List()

[Nils Goroll]

commit 4ac5e49f558096a68861ef2bfd975727ec364922
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Thu Apr 19 08:41:50 2018 +0200

    don't panic for ws overflow in VBF_Get_Filter_List()

diff --git a/bin/varnishd/cache/cache_fetch.c b/bin/varnishd/cache/cache_fetch.c
index 35e7d25..07259f6 100644
--- a/bin/varnishd/cache/cache_fetch.c
+++ b/bin/varnishd/cache/cache_fetch.c
@@ -605,7 +605,8 @@ vbf_stp_fetch(struct worker *wrk, struct busyobj *bo)
 		bo->filter_list = VBF_Get_Filter_List(bo);
 	}
 
-	if (VFP_FilterList(bo->vfc, bo->filter_list)) {
+	if (bo->filter_list == NULL ||
+	    VFP_FilterList(bo->vfc, bo->filter_list)) {
 		(bo)->htc->doclose = SC_OVERLOAD;
 		VDI_Finish((bo)->wrk, bo);
 		return (F_STP_ERROR);