mds/Migrator.cc: fix possible dereference NULL return value

mds/Migrator.cc: fix possible dereference NULL return value

ID 716998 (#1 of 2): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "in" when
  calling "operator <<(std::ostream &, CInode &)".

CID 716998 (#2 of 2): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "in" when
  calling "MDCache::add_replica_dir(ceph::buffer::list::iterator &,
  CInode *, int, std::list<Context *, std::allocator<Context *> > &)".

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>