Fix crash from ~VisualizerProtocol() throwing an exception.
Fix crash from ~VisualizerProtocol() throwing an exception.

Previously, if the simbody-visualizer GUI was closed before
`VisualizerProtocol::stopListeningIfNecessary()` was called, the
simulator would crash because writing to the visualizer pipe caused an
exception to be thrown in `~VisualizerProtocol()`.

simbody/simbody,simbody/simbody