zeros(AffExpr) not safe

[klamike]

https://discourse.julialang.org/t/why-are-the-entries-identical-in-zeros-jump-affexpr-n/130860?u=klamike

[klamike]

Actually I think we're fine since we never actually use the mutable API, just do that to initialize the container of correct length. Still should be adjusted if we stick with this way of generating the rules