loose: avoid closing invalid fd on error path

`write_one_object()` opens a file at line 186 and jumps to the errout
label on failure. The errout cleanup unconditionally calls `close(fd)`,
but when `open()` itself failed, fd is -1. Calling `close(-1)` is
harmless on most platforms (returns EBADF) but is undefined behavior per
POSIX and can confuse fd tracking in sanitizer builds.

Guard the close with fd >= 0.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

loose.c

@@ -201,7 +201,8 @@ static int write_one_object(struct odb_source_loose *loose,
 	return 0;
 errout:
 	error_errno(_("failed to write loose object index %s"), path.buf);
-	close(fd);
+	if (fd >= 0)
+		close(fd);
 	rollback_lock_file(&lock);
 	strbuf_release(&buf);
 	strbuf_release(&path);