scsi: mvsas: mv_sas.c: Fix for possible null pointer dereference

There is otherwise a risk of a possible null pointer dereference.

Was largely found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>