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>