Fixes a crash on unload if the sync_tick callback was running at the same time
the dynamic local span was destroyed. It was possible for
dahdi_dynamic_local_transmit to dereference a pointer that may have already
been freed.
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@10627 a0bf4364-ded3-4de4-8d8a-66a801d63aff