Fix surplus vcldir cleanup

in VRT_AddDirector, we create the new vcldir with an initial
reference, which we need to drop if we can not add it.

Compare:

	VRT_AddDirector()
		...
		vdir->refcnt++;

	vcldir_free()
		...
		AZ(vdir->refcnt);

Noticed when testing other experimental changes while working on
https://github.com/nigoroll/libvmod-dynamic/issues/110

 #5  0x000055820c8cb845 in VAS_Fail (func=0x55820c904559 "vcldir_free", file=0x55820c903a47 "cache/cache_vrt_vcl.c",
     line=150, cond=0x55820c90459a "(vdir->refcnt) == 0", kind=VAS_ASSERT) at vas.c:67
 #6  0x000055820c83a442 in vcldir_free (vdir=0x7f662aa53140) at cache/cache_vrt_vcl.c:150
 #7  0x000055820c839fe1 in VRT_AddDirector (ctx=0x7f662befe250, m=0x55820c965260 <vbe_methods_noprobe>,
     priv=0x7f662aa20780, fmt=0x55820c900f7f "%s") at cache/cache_vrt_vcl.c:219
 #8  0x000055820c7c7c4d in VRT_new_backend_clustered (ctx=0x7f662befe250, vc=0x0, vrt=0x7f662befdd10, via=0x0)
    at cache/cache_backend.c:737
 #9  0x000055820c7c8632 in VRT_new_backend (ctx=0x7f662befe250, vrt=0x7f662befdd10, via=0x0)
     at cache/cache_backend.c:755

bin/varnishd/cache/cache_vrt_vcl.c

@@ -218,6 +218,8 @@ VRT_AddDirector(VRT_CTX, const struct vdi_methods *m, void *priv,
 	Lck_Unlock(&vcl_mtx);
 
 	if (temp == VCL_TEMP_COOLING) {
+		assert(vdir->refcnt == 1);
+		vdir->refcnt = 0;
 		vcldir_free(vdir);
 		return (NULL);
 	}