ceph-dencoder: Silence coverity CID 1412579

ceph-dencoder: Silence coverity CID 1412579

141         if (i == 0)
142           i = m_list.size();
143         if ((i == 0) || (i > m_list.size()))
144           return "invalid id for generated object";
145         typename list<T*>::iterator p = m_list.begin();
146         for (i--; i > 0 && p != m_list.end(); ++p, --i) ;
>>>     CID 1412579:  API usage errors  (INVALIDATE_ITERATOR)
>>>     Dereferencing iterator "p" though it is already past the end
of its container.
147         m_object = *p;
148         return string();
149       }
150
151       bool is_deterministic() override {
152         return !nondeterministic;

Signed-off-by: Brad Hubbard <bhubbard@redhat.com>