mds/Server.cc: fix possible NULL pointer dereference

CID 1019554 (#2 of 2): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "straydn" to function
  "MDSCacheObject::is_auth() const", which dereferences it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
src/dupstore.cc: check return value of list_collections()

CID 1019545 (#1 of 1): Unchecked return value (CHECKED_RETURN)
  check_return: Calling function "ObjectStore::list_collections
  (std::vector<coll_t, std::allocator<coll_t> > &)" without
  checking return value (as is done elsewhere 5 out of 6 times).

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