[master] b5a29eb6a Appease coverity

[Federico G. Schwindt]

commit b5a29eb6a8d3ff39d1db4ee18af8bd0373796195
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Thu Mar 14 23:12:49 2019 +0000

    Appease coverity

diff --git a/bin/varnishtest/vtc_haproxy.c b/bin/varnishtest/vtc_haproxy.c
index 7a54a7b0b..a3a5ed21e 100644
--- a/bin/varnishtest/vtc_haproxy.c
+++ b/bin/varnishtest/vtc_haproxy.c
@@ -832,6 +832,7 @@ haproxy_write_conf(struct haproxy *h)
 
 	vsb = macro_expand(h->vl, VSB_data(h->cfg_vsb));
 	AN(vsb);
+	assert(VSB_len(vsb) >= 0);
 
 	vtc_dump(h->vl, 4, "conf", VSB_data(vsb), VSB_len(vsb));
 	if (VFIL_writefile(h->workdir, h->cfg_fn,