Add correct IDisposable implementation

Improvement #36 (closed)

Once an item has been disposed and freed on the native side, if the user keeps a reference on that object and perform calls (with a NativeReference equals to IntPtr.Zero), it will cause an AccessViolationException.

Granted the user should not do that, but our code should not crash either. Should we perform a NativeReference check before each libvlc call to avoid this exception when users perform calls on disposed instances? And output to log if this happens.

cc @jeremyVignelles