stream: fix decoded fromList chunk boundary check

Correct `fromList()` in decoded string mode to compare `n` against the
current chunk length, not the buffer array length.

This prevents over-consuming chunks, which can corrupt readable state
and crash with `TypeError` when mixing `setEncoding()` and `read(n)`.

PR-URL: #61884
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Ilyas Shabi <ilyasshabi94@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>

Author: watson

lib/internal/streams/readable.js

@@ -1633,7 +1633,7 @@ function fromList(n, state) {
         n -= str.length;
         buf[idx++] = null;
       } else {
-        if (n === buf.length) {
+        if (n === str.length) {
           ret += str;
           buf[idx++] = null;
         } else {

test/parallel/test-stream2-read-correct-num-bytes-in-utf8.js

@@ -0,0 +1,16 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const { Readable } = require('stream');
+
+const readable = new Readable({ read() {} });
+readable.setEncoding('utf8');
+
+readable.push('abc');
+readable.push('defgh');
+readable.push(null);
+
+assert.strictEqual(readable.read(5), 'abcde');
+assert.strictEqual(readable.read(3), 'fgh');
+assert.strictEqual(readable.read(1), null);