stream: simplify src._readableState to state

PR-URL: #18264
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: 陈刚

lib/_stream_readable.js

@@ -645,8 +645,8 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
       if (((state.pipesCount === 1 && state.pipes === dest) ||
            (state.pipesCount > 1 && state.pipes.indexOf(dest) !== -1)) &&
           !cleanedUp) {
-        debug('false write response, pause', src._readableState.awaitDrain);
-        src._readableState.awaitDrain++;
+        debug('false write response, pause', state.awaitDrain);
+        state.awaitDrain++;
         increasedAwaitDrain = true;
       }
       src.pause();