I've spotted this while working on something else.

OK?

diff --git lib/libc/net/res_comp.c lib/libc/net/res_comp.c
index ce9f92ae771..43d50f93c2b 100644
--- lib/libc/net/res_comp.c
+++ lib/libc/net/res_comp.c
@@ -348,6 +348,9 @@ __res_hnok(const char *dn)
 {
 	int pch = PERIOD, ch = *dn++;
 
+	if (ch == '\0')
+		return (0);
+
 	while (ch != '\0') {
 		int nch = *dn++;
 

-- 
In my defence, I have been left unsupervised.