Avoid accessing a trie node outside of the critical region.

[shinji-s]

It is thread-unsafe to dereference a pointer to a trie node outside of the critical region protected by the mutex because another thread may delete the node before the dereference, isn't it? (This PR supercedes https://github.com/ecmwf/eccodes/pull/38.)

[shinji-s]

Failure of travis-ci test is due to a failure in downloading test data. This happens at higher probability recently.