mosta: dot sometimes fails with full state labels

When running
mosta "Case Studies\pta\brp.modest" -O out.dot -D png out.png -Y --no-transient-values -E
dot does not produce an output png file and mosta consequently crashes. With -S, it works.

1. Fix the crash.
2. Check if the reason for dot failure is in the dot file generated by mosta, and if yes, fix it.