mds/MDCache.cc: fix dereference after null check

Add assert for 'parent' before call assert on parent->is_auth().

CID 716922 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "parent" to function
  "MDSCacheObject::is_auth() const", which dereferences it.

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