Should not allow 'end' events to happen more than once

Likely root cause of:
zkat/pacote#151
isaacs/node-tar#180
laravel/homestead#866

index.js

@@ -265,20 +265,22 @@ module.exports = class MiniPass extends EE {
       if (this.pipes.length)
         this.pipes.forEach(p => p.dest.write(data) || this.pause())
     } else if (ev === 'end') {
+      this[EMITTED_END] = true
+      this.readable = false
+
       if (this[DECODER]) {
         data = this[DECODER].end()
         if (data) {
           this.pipes.forEach(p => p.dest.write(data))
           super.emit('data', data)
         }
       }
+
       this.pipes.forEach(p => {
         p.dest.removeListener('drain', p.ondrain)
         if (!p.opts || p.opts.end !== false)
           p.dest.end()
       })
-      this[EMITTED_END] = true
-      this.readable = false
     } else if (ev === 'close') {
       this[CLOSED] = true
       // don't emit close before 'end' and 'finish'

test/end-twice.js

@@ -0,0 +1,21 @@
+'use strict'
+const t = require('tap')
+const MP = require('../')
+
+const butterfly = Buffer.from([0x61, 0xf0, 0x9f, 0xa6, 0x8b, 0xf0])
+const mp = new MP({ encoding: 'utf8' })
+
+let sawEnd = 0
+mp.on('end', () =>
+  t.equal(sawEnd++, 0, 'should not have seen the end yet'))
+
+mp.once('data', () => {
+  mp.once('data', () => {
+    mp.once('data', () => mp.end())
+    mp.end()
+  })
+  mp.end(butterfly.slice(3))
+})
+mp.end(butterfly.slice(0, 3))
+
+t.equal(sawEnd, 1, 'should see end exactly once')