Declared statics to be thread safe in MiniFloatConverter

The statics are filled at library load time but can't be made const
because of the complexity of filling arrays. We demark them as
thread safe to avoid the false positive from the static analyzer.
Merge pull request #3622 from Dr15Jones/declareStaticsThreadSafeInMiniFloatConverter

Multithreading -- Declared statics to be thread safe in MiniFloatConverter