Is this cracked rear derailleur safe to use So I've been building up a late 80's steel frame to a modern 11 speed, but still using downtube friction shifters. After doing a few runs up and down my street all felt OK so I decided to take it out on it's maiden voyage.

A couple of miles down the road however something didn't feel right, after pulling in to a cul de sac to try and see what was wrong I was testing going up and down the gears a few times when the pedals jammed. Luckily I was going quite slow and managed to jump off. Anyway it appears one of the jockey wheels became jammed between the spokes. I'm guessing I may have offset the H limit screw too far or something?

Anyway the damage is it's cracked the housing of the rear derailleur hanging bolt (see below). Is this safe to use or should I get a new one?
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.