debug/macho: use bytes.IndexByte instead of a loop

Simpler, and no doubt faster.

Change-Id: Idd401918da07a257de365087721e9ff061e6fd07
Reviewed-on: https://go-review.googlesource.com/98759
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/macho/file.go

@@ -545,8 +545,9 @@ func (f *File) pushSection(sh *Section, r io.ReaderAt) error {
 }
 
 func cstring(b []byte) string {
-	var i int
-	for i = 0; i < len(b) && b[i] != 0; i++ {
+	i := bytes.IndexByte(b, 0)
+	if i == -1 {
+		i = len(b)
 	}
 	return string(b[0:i])
 }