Initialize a stack variable in mprsas_get_sas_address_for_sata_disk

Thought it's difficult to reproduce, I think this variable was responsible
for a use-after-free panic when a SATA disk timed out responding to a SATA
identify command during boot.

Submitted by: slm
Reviewed by: slm
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D9364