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>