When returning error from msdosfs_lookup(), make sure that *vpp is NULL. lookup() KASSERTs this condition.
When returning error from msdosfs_lookup(), make sure that *vpp is NULL.
lookup() KASSERTs this condition.

Reported and tested by:	pho
MFC after:	3 weeks

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