mds/Server.cc: fix possible NULL pointer dereference

CID 1019557 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "destdn" to function
  "CDentry::get_dir() const", which dereferences it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
mds/Server.cc: fix possible NULL pointer dereference

CID 1019555 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "in" to function
  "Server::_need_force_journal(CInode *, bool)", which dereferences it.

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