fix an off-by one error: failing assertion if only one backend configured

src/tests/v01009.vtc

+varnishtest "VSLP - single backend works"
+
+server s1 {
+	rxreq
+	txresp -body "ech3Ooj"
+} -start
+
+varnish v1 -vcl+backend {
+	import vslp from "${vmod_topbuild}/src/.libs/libvmod_vslp.so" ;
+
+	sub vcl_init {
+		new vd = vslp.vslp();
+		vd.add_backend(s1);
+		vd.set_rampup_ratio(0);
+		vd.init_hashcircle(25);
+	}
+
+	sub vcl_recv {
+		set req.backend_hint = vd.backend();
+		return(pass);
+	}
+
+} -start
+
+
+client c1 {
+	txreq -url /eishoSu2
+	rxresp
+	expect resp.body == "ech3Ooj"
+} -run

src/vslp_dir.c

@@ -304,7 +304,7 @@ vslpdir_init_hashcircle(struct vslpdir *vslpd, VCL_INT replicas)
 		sizeof(struct vslp_hostnode));
 	AN(vslpd->hashcircle);
 	AN(vslpd->backend);
-	if(vslpd->backend[1] == NULL)
+	if(vslpd->backend[0] == NULL)
 	{
 		vslpdir_unlock(vslpd);
 		WRONG("VSLP director doesn't have any backends");