Refinement on previous fix for mutex destruction: make sure we don't release the mutex multiple times for multi-plex volumes.
Refinement on previous fix for mutex destruction: make sure we don't
release the mutex multiple times for multi-plex volumes.

Following further consultation with:	grog

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase