Properly handle empty address string instead of panic

ntp.go

@@ -648,6 +648,10 @@ func dialWrapper(la, ra string,
 // fixHostPort examines an address in one of the accepted forms and fixes it
 // to include a port number if necessary.
 func fixHostPort(address string, defaultPort int) (fixed string, err error) {
+	if len(address) == 0 {
+		return "", errors.New("address string is empty")
+	}
+
 	// If the address is wrapped in brackets, append a port if necessary.
 	if address[0] == '[' {
 		end := strings.IndexByte(address, ']')

ntp_test.go

@@ -270,6 +270,7 @@ func TestOfflineFixHostPort(t *testing.T) {
 		{"[::ffff:192.168.1.1]", "[::ffff:192.168.1.1]:123", ""},
 		{"[::ffff:192.168.1.1]:123", "[::ffff:192.168.1.1]:123", ""},
 		{"[::ffff:192.168.1.1]:1000", "[::ffff:192.168.1.1]:1000", ""},
+		{"", "", "address string is empty"},
 	}
 	for _, c := range cases {
 		fixed, err := fixHostPort(c.address, defaultPort)