net: fix hostLookupOrder("")

Fixes #13623.

Change-Id: I1bd96aa7b6b715e4dbdcf0c37c2d29228df6565c
Reviewed-on: https://go-review.googlesource.com/18329Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/conf.go

@@ -220,7 +220,7 @@ func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
 	var first string
 	for _, src := range srcs {
 		if src.source == "myhostname" {
-			if hasDot {
+			if hostname == "" || hasDot {
 				continue
 			}
 			return fallbackOrder

src/net/conf_test.go

@@ -257,6 +257,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupFilesDNS},
 				{"somehostname", hostLookupCgo},
+				{"", hostLookupFilesDNS}, // Issue 13623
 			},
 		},
 		{