Fix `multiway`'s behavior when dealing with zero-valued axis of `speed`.

This fixes the behavior of `multiway` when `speed` is as an object
with 0 value for either axis. Passing `{x: 0, y: 0}`, or variations
of, is now safe.
Merge pull request #516 from mnemotic/speed-fix

Fix `multiway`'s behavior when dealing with zero-valued axis of `speed`.