os/exec: LookPath on Unix shouldn't look in cwd when PATH is empty

R=golang-dev, dave, bradfitz
CC=golang-dev
https://golang.org/cl/7305053

src/pkg/os/exec/lp_unix.go

@@ -42,6 +42,9 @@ func LookPath(file string) (string, error) {
 		return "", &Error{file, err}
 	}
 	pathenv := os.Getenv("PATH")
+	if pathenv == "" {
+		return "", &Error{file, ErrNotFound}
+	}
 	for _, dir := range strings.Split(pathenv, ":") {
 		if dir == "" {
 			// Unix shell semantics: path element "" means "."