This PR prevents memory exhaustion turning into segfaults when using
Lean functions which call into libuv
`malloc` can return `NULL`, in which case this code would previously go
on to dereference a null pointer.
Instead, it now returns a suitable `IO.Error`.
Calling `lean_internal_panic_out_of_memory` would also be an option
here, since the adjacent `lean_promise_new` calls would fail in this
way.
6819c805b9