Asserting that an entity has a MOC

I'll save the next poor sucker the hassle of diagnosing this the long way.

(For those playing at home, in my case, when `GCC_OPTIMIZATION_LEVEL` == `0`, it worked fine. I was only getting errors in prod builds. Don't be a fool like me, and keep a reference to your MOC so you can, you know, do work within it.)