syscall: optimize Getwd on aix

When looking for \0, use clen which may be optimized.

Also, return EINVAL when returned string is empty.

This makes it similar to how it is implemented in *bsd and solaris.

Change-Id: I3e37ed25f47110eafd12c80291f7746de9db7b23
Reviewed-on: https://go-review.googlesource.com/c/go/+/606902


TryBot-Bypass: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>

src/syscall/syscall_aix.go

@@ -119,11 +119,11 @@ func Getwd() (ret string, err error) {
 		b := make([]byte, len)
 		err := getcwd(&b[0], len)
 		if err == nil {
-			i := 0
-			for b[i] != 0 {
-				i++
+			n := clen(b[:])
+			if n < 1 {
+				return "", EINVAL
 			}
-			return string(b[0:i]), nil
+			return string(b[:n]), nil
 		}
 		if err != ERANGE {
 			return "", err