loose: avoid closing invalid fd on error path

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>