scsi: megaraid: Fix mega_cmd_done() CMDID_INT_CMDS

When cmdid == CMDID_INT_CMDS, the 'cmds' pointer is NULL but is
dereferenced below.

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

Fixes: 0f2bb84d

 ("[SCSI] megaraid: simplify internal command handling")
Signed-off-by: Danila Chernetsov <listdansp@mail.ru>
Link: https://lore.kernel.org/r/20230317175109.18585-1-listdansp@mail.ru
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>