buffer: fix MAX_LENGTH constant export

This was a typo and accidentally returned the wrong value.

Fixes: nodejs#14819
Ref: nodejs#13467

Author: addaleax

lib/buffer.js

@@ -76,7 +76,7 @@ Buffer.prototype = FastBuffer.prototype;
 
 const constants = Object.defineProperties({}, {
   MAX_LENGTH: {
-    value: kStringMaxLength,
+    value: kMaxLength,
     writable: false,
     enumerable: true
   },

test/parallel/test-buffer-constants.js

@@ -2,6 +2,7 @@
 require('../common');
 const assert = require('assert');
 
+const { kMaxLength, kStringMaxLength } = require('buffer');
 const { MAX_LENGTH, MAX_STRING_LENGTH } = require('buffer').constants;
 
 assert.strictEqual(typeof MAX_LENGTH, 'number');
@@ -11,3 +12,7 @@ assert.throws(() => ' '.repeat(MAX_STRING_LENGTH + 1),
               /^RangeError: Invalid string length$/);
 
 assert.doesNotThrow(() => ' '.repeat(MAX_STRING_LENGTH));
+
+// Legacy values match:
+assert.strictEqual(kMaxLength, MAX_LENGTH);
+assert.strictEqual(kStringMaxLength, MAX_STRING_LENGTH);