misc minor fixes in mpr(4)

sys/dev/mpr/mpr_sas.c

• Fix a potential null pointer dereference (CID 1305731)
• Check for overrun of the ccb_scsiio.cdb_io.cdb_bytes buffer (CID 1211934)

sys/dev/mpr/mpr_sas_lsi.c

• Nullify a dangling pointer in mprsas_get_sata_identify
• Fix a memory leak in mprsas_SSU_to_SATA_devices (CID 1211935)

Reported by: Coverity (partially)
CID: 1305731 1211934 1211935
Reviewed by: slm
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D8880