fix vcl temperature panic output

When we turned the vcl temperatures into a struct, we overlooked this
statement because of the cast.

Noticed when staring at #3192

bin/varnishd/cache/cache_vcl.c

@@ -222,7 +222,7 @@ VCL_Panic(struct vsb *vsb, const char *nm, const struct vcl *vcl)
 	VSB_printf(vsb, "busy = %u,\n", vcl->busy);
 	VSB_printf(vsb, "discard = %u,\n", vcl->discard);
 	VSB_printf(vsb, "state = %s,\n", vcl->state);
-	VSB_printf(vsb, "temp = %s,\n", (const volatile char *)vcl->temp);
+	VSB_printf(vsb, "temp = %s,\n", vcl->temp ? vcl->temp->name : "(null)");
 	VSB_cat(vsb, "conf = {\n");
 	VSB_indent(vsb, 2);
 	if (vcl->conf == NULL) {