work around compiler quirk

Author: Alexander Early

lib/internal/awaitify.js

@@ -1,6 +1,7 @@
 // conditionally promisify a function.
 // only return a promise if a callback is omitted
-export default function awaitify (asyncFn, arity = asyncFn.length) {
+export default function awaitify (asyncFn, arity) {
+    if (!arity) arity = asyncFn.length;
     if (!arity) throw new Error('arity is undefined')
     function awaitable (...args) {
         if (typeof args[arity - 1] === 'function') {