Fix for a crash seen at PlugFest 39 (CVP2RI-402)

The implementation of stop() for a mp4 container stream sets
 this.mp4_container_stream/mp4_container_source to null after stop.
 If/when the mp4 container push thread catches the exception from the
 stopped BufferGeneratingOutputStream, it can access the nulled
 variables causing an access violation, depending on which thread runs
 first.

Fixed the issue by extracting the container source name at the top of the
 container push thread function and putting it into a local variable.

Still need to retest the scenario with the vendor to validate the crash
 is fixed.