s3: Fix a crash in reply_lockingX_error

s3: Fix a crash in reply_lockingX_error

A timed brlock with 2 locks comes in and the second one blocks,
file is closed. smbd_cancel_pending_lock_requests_by_fid sets
blr->fsp to NULL. reply_lockingX_error (called via
MSG_SMB_BLOCKING_LOCK_CANCEL) deferences blr->fsp because
blr->lock_num==1 (the second one blocked).

This patch fixes the bug by only undoing the locks if fsp!=NULL.
fsp==NULL is the close case where everything is undone anyway.

Thanks to Peter Somogyi, somogyi@hu.ibm.com for this bug report.

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Aug  8 04:12:04 CEST 2012 on sn-devel-104