fix: don't close while writing

Fixes: #98

index.js

@@ -5,6 +5,7 @@ const EventEmitter = require('events')
 const inherits = require('util').inherits
 const path = require('path')
 const sleep = require('atomic-sleep')
+const assert = require('assert')
 
 const BUSY_WRITE_TIMEOUT = 100
 
@@ -374,6 +375,11 @@ function actualClose (sonic) {
     sonic.once('ready', actualClose.bind(null, sonic))
     return
   }
+  if (sonic._writing) {
+    sonic.once('drain', actualClose.bind(null, sonic))
+    return
+  }
+  assert(!sonic._writing)
   // TODO write a test to check if we are not leaking fds
   fs.close(sonic.fd, (err) => {
     if (err) {