verify_signed_buffer: prefer close_tempfile() to close()

verify_signed_buffer: prefer close_tempfile() to close()

We do a manual close() on the descriptor provided to us by
mks_tempfile. But this runs contrary to the advice in
tempfile.h, which notes that you should always use
close_tempfile(). Otherwise the descriptor may be reused
without the tempfile object knowing it, and the later call
to delete_tempfile() could close a random descriptor.

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