scsi: 53c700: Check that command slot is not NULL

Add a check for the command slot value to avoid dereferencing a NULL
pointer.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Co-developed-by: Vladimir Telezhnikov <vtelezhnikov@astralinux.ru>
Signed-off-by: Vladimir Telezhnikov <vtelezhnikov@astralinux.ru>
Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
Link: https://lore.kernel.org/r/20230728123521.18293-1-adiupina@astralinux.ru


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>