stream: Never call decoder.end() multiple times

Fixes decoder.end() being called on every push(null). As the tls module
does this, corrupt stream data could potentially be added to the end.

lib/_stream_readable.js

@@ -349,6 +349,7 @@ function onEofChunk(stream, state) {
       state.buffer.push(chunk);
       state.length += state.objectMode ? 1 : chunk.length;
     }
+    state.decoder = null;
   }
 
   // if we've ended and we have some data left, then emit

test/simple/test-stream2-set-encoding.js

@@ -163,6 +163,10 @@ test('setEncoding hex', function(t) {
 
   // just kick it off.
   tr.emit('readable');
+
+  process.on('exit', function() {
+    assert(!tr._readableState.decoder);
+  });
 });
 
 test('setEncoding hex with read(13)', function(t) {