Release the GIL on shutdown only if it can be released
Release the GIL on shutdown only if it can be released

Fixes an issue where there is no current thread state on shutdown.
Also fixes a comment in Runtime.Initialize

AlexCatarino/pythonnet,AlexCatarino/pythonnet,QuantConnect/pythonnet,pythonnet/pythonnet,AlexCatarino/pythonnet,pythonnet/pythonnet,QuantConnect/pythonnet,QuantConnect/pythonnet,AlexCatarino/pythonnet,pythonnet/pythonnet