mds/Server.cc: fix possible NULL pointer dereference

Add asserts to solve these CID issues:

CID 717002 (#1 of 2): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "dir"
  when calling "CDir::lookup(std::string const &, snapid_t)".
CID 717002 (#2 of 2): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "dir"
  when calling "CDir::lookup(std::string const &, snapid_t)".

CID 717003 (#1 of 2): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "dn" when
  calling "operator <<(std::ostream &, CDentry &)"
CID 717003 (#2 of 2): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "straydn"
  when calling "CDentry::push_projected_linkage()".

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