Fix race condition. If multiple runMALdataflow's are called simultaneously all may end up calling DFLOWinitialize().  If one is slightly ahead and initializes todo, and the next then tests todo, and returns, it may hit the assert(workers[0]) before the first thread forked off the first worker thread.
Fix race condition.
If multiple runMALdataflow's are called simultaneously all may end up
calling DFLOWinitialize().  If one is slightly ahead and initializes
todo, and the next then tests todo, and returns, it may hit the
assert(workers[0]) before the first thread forked off the first worker
thread.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb