Is this cracked rear derailleur safe to use
Define safe.  Are you just worried it will fail or that you will get hurt?  

It is going to fail all the way and sooner rather than later. 

If it jams into the rear wheel it might damage other stuff (like spokes).

The safe bet is to just replace it now.  

If it jams into the rear wheel it might cause you to get hurt.  If pedals jammed might cause you to fall then yes in that respect you could get hurt.  

Save it for parts.