http-push: avoid invalid memory accesses

http-push: avoid invalid memory accesses

Before objects are sent, the respective ref is locked.  However,
without this patch, the lock is lifted before the last object for
that ref was sent.  As a consequence, the lock data was accessed
after the lock structure was free()d.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>