diff-no-index: release strbuf on queue error

diff-no-index: release strbuf on queue error

The strbuf is small and we are about to exit, so we could leave its
cleanup to the OS.  If we release it explicitly at all, however, then we
should do it on early exit as well.  Move the strbuf_release call to a
new cleanup section at the end and make sure all execution paths go
through it.

Suggested-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>