mds: fix coverity warnings on NULL deref

mds: fix coverity warnings on NULL deref

Add asserts...

At (5): Function "MDCache::get_dirfrag(dirfrag_t)" returns null (checked 33 out of 39 times). [show details]
At (6): Assigning: "dir" = null return value from "MDCache::get_dirfrag(dirfrag_t)".
CID 717007 (#1 of 1): Dereference null return value (NULL_RETURNS)
At (7): Dereferencing a pointer that might be null "dir" when calling "MDCache::adjust_bounded_subtree_auth(CDir *, std::vector<dirfrag_t, std::allocator<dirfrag_t> > &, std::pair<int, int>)". [show details]

CID 717006 (#1 of 1): Dereference null return value (NULL_RETURNS)
At (5): Dereferencing a pointer that might be null "dir" when calling "MDCache::adjust_bounded_subtree_auth(CDir *, std::vector<dirfrag_t, std::allocator<dirfrag_t> > &, std::pair<int, int>)". [show details]

CID 717005 (#2 of 2): Dereference null return value (NULL_RETURNS)
At (22): Dereferencing a pointer that might be null "dir" when calling "MDCache::adjust_bounded_subtree_auth(CDir *, std::vector<dirfrag_t, std::allocator<dirfrag_t> > &, int)". [show details]

Signed-off-by: Sage Weil <sage@inktank.com>