check for both null AND undefined in stream writer (#44)

This fixes IDZ support when minime is run using a newer LTS version of
node js.
(Streams use null for the error argument now for write and end
callbacks)

src/util/stream/writer.ts

@@ -14,7 +14,7 @@ export default function makeWriter(stm: Writable) {
       stm.write(buf, error => {
         busy = false;
 
-        if (error !== undefined) {
+        if (error !== null && error !== undefined) {
           reject(error);
         } else {
           resolve();