Avoid exception in Visualizer::Impl::~Impl().
Avoid exception in Visualizer::Impl::~Impl().

This exception, which occurred when destructing a model whose
simbody-visualizer was already shutdown, was causing OpenSim's MATLAB and
Python bindings to crash.

simbody/simbody,Gjacquenot/simbody,Gjacquenot/simbody,simbody/simbody,Gjacquenot/simbody