Log what we actually tried to use

bin/varnishd/cache/cache_http.c

@@ -1210,8 +1210,9 @@ http_PrintfHeader(struct http *to, const char *fmt, ...)
 	va_end(ap);
 	if (n + 1 >= l || to->nhd >= to->shd) {
 		http_fail(to);
-		VSLb(to->vsl, SLT_LostHeader, "%s",
-		    n + 1 >= l ? fmt : to->ws->f);
+		va_start(ap, fmt);
+		VSLbv(to->vsl, SLT_LostHeader, fmt, ap);
+		va_end(ap);
 		WS_Release(to->ws, 0);
 		return;
 	}