Fix for EXT-4207 (Viewer crash clicking on an object that starts a script that...

Fix for EXT-4207 (Viewer crash clicking on an object that starts a script that navigates in a while(1) loop)

Don't allow LLViewerMediaImpl to process a navigateInternal during the call to LLPluginClassMedia::idle().  This prevents untimely destruction of the LLPluginClassMedia and LLPluginMessagePipe objects, which was causing the crash.

Reviewed by Rick.