[SCSI] sym53c8xx: Fix NULL pointer dereference in slave_destroy

sym53c8xx_slave_destroy unconditionally assumes that sym53c8xx_slave_alloc has
succesesfully allocated a sym_lcb. This can lead to a NULL pointer dereference
(exposed by commit 4e6c82b3).

Signed-off-by: Stratos Psomadakis <psomas@gentoo.org>
Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>