[investigate] wrenCall arity check

[ruby0x1]

While exploring something else I noticed this, the closure passed to wrenCall checks arity but it's 0 in the cases I've seen.
#1124 is likely related, but good to add tests for when there.

 ASSERT(vm->fiber->stackTop - vm->fiber->stack >= closure->fn->arity,
        "Stack must have enough arguments for method.");

[mhermier]

Related to #731