scsi: sym53c8xx: fix NULL pointer dereference panic in sym_int_sir()

sym_int_sir() in sym_hipd.c does not check the command pointer for NULL before
using it in debug message prints.

Suggested-by: Matthew Wilcox <matthew.wilcox@oracle.com>
Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Acked-by: Matthew Wilcox <matthew.wilcox@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>