User may do "mount -o snapshot ...", that causes new FFS mount to be performed with snapshot option, while the mp->mnt_opt is NULL. Protect against NULL pointer dereference.
User may do "mount -o snapshot ...", that causes new FFS mount to be
performed with snapshot option, while the mp->mnt_opt is NULL.
Protect against NULL pointer dereference.

Noted by:	Mateusz Guzik <mjguzik gmail com>
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase