done_ccb pointer can be used if it is NULL.

To prevent this, move check for done_ccb == NULL to before done_ccb is used in
mprsas_stop_unit_done().

Reviewed by: ken, scottl, ambrisko, asomers
Approved by: ken, scottl, ambrisko
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6099