MUSIC runs into an endless loop if running just one process

[weidel-p]

As mentioned in nest/nest-simulator#958 MUSIC does not finish it's simulation if there is only one process defined in the MUSIC configuration file. Apparently, the function pushForward in the scheduler runs into an endless loop in this case.

This is not a major error as there is no reason to run music with only one process, but I guess it would be nice to fix it anyways.