strbuf_read_file(): preserve errno across close() call

If we encounter a read error, the user may want to report it
by looking at errno. However, our close() call may clobber
errno, leading to confusing results. Let's save and restore
it in the error case.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>