mds/Migrator.cc: fix dereference NULL return value
CID 717000 (#1 of 2): Dereference null return value (NULL_RETURNS)
dereference: Dereferencing a pointer that might be null "dir" when
calling "operator <<(std::ostream &, CDir &)".
CID 717000 (#2 of 2): Dereference null return value (NULL_RETURNS)
dereference: Dereferencing a pointer that might be null "dir" when
calling "Migrator::import_reverse_unfreeze(CDir *)".
Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>