syscall: fix exec_bsd.go to accompany exec_linux.go changes

exec_plan9.go too.
Those are in CL 8334044

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/9055043

src/pkg/syscall/exec_bsd.go

@@ -39,10 +39,18 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		i      int
 	)
 
+	// guard against side effects of shuffling fds below.
+	// Make sure that nextfd is beyond any currently open files so
+	// that we can't run the risk of overwriting any of them.
 	fd := make([]int, len(attr.Files))
+	nextfd = len(attr.Files)
 	for i, ufd := range attr.Files {
+		if nextfd < int(ufd) {
+			nextfd = int(ufd)
+		}
 		fd[i] = int(ufd)
 	}
+	nextfd++
 
 	darwin := runtime.GOOS == "darwin"
 
@@ -131,7 +139,6 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 
 	// Pass 1: look for fd[i] < i and move those up above len(fd)
 	// so that pass 2 won't stomp on an fd it needs later.
-	nextfd = int(len(fd))
 	if pipe < nextfd {
 		_, _, err1 = RawSyscall(SYS_DUP2, uintptr(pipe), uintptr(nextfd), 0)
 		if err1 != 0 {