streams: fix regression in unpipe()

Since 2e568d9 there is a bug where unpiping a stream
from a readable stream that has `_readableState.pipesCount > 1`
will cause it to remove the first stream in the
`_.readableState.pipes` array no matter where in the list the
`dest` stream was.

This patch corrects that problem.

Fixes: #9170

lib/_stream_readable.js

@@ -671,7 +671,7 @@ Readable.prototype.unpipe = function(dest) {
   if (index === -1)
     return this;
 
-  state.pipes.splice(i, 1);
+  state.pipes.splice(index, 1);
   state.pipesCount -= 1;
   if (state.pipesCount === 1)
     state.pipes = state.pipes[0];