os: add check for empty executable path on darwin

os used to panic (access out of bounds) if executablePath was left empty

Fixes #22529

Change-Id: Iead5e60a3b000dbde421a8e8612c3690340879ce
Reviewed-on: https://go-review.googlesource.com/127546
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/executable_darwin.go

@@ -4,12 +4,17 @@
 
 package os
 
+import "errors"
+
 var executablePath string // set by ../runtime/os_darwin.go
 
 var initCwd, initCwdErr = Getwd()
 
 func executable() (string, error) {
 	ep := executablePath
+	if len(ep) == 0 {
+		return ep, errors.New("cannot find executable path")
+	}
 	if ep[0] != '/' {
 		if initCwdErr != nil {
 			return ep, initCwdErr