buffer: stricter argument checking in toString

This prevents the confusing behavior of `buf.toString(0, 5)` by
disallowing passing `0` as the encoding.

PR-URL: https://github.com/nodejs/node/pull/11120
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/buffer.js

@@ -463,7 +463,7 @@ function slowToString(encoding, start, end) {
   if (end <= start)
     return '';
 
-  if (!encoding) encoding = 'utf8';
+  if (encoding === undefined) encoding = 'utf8';
 
   while (true) {
     switch (encoding) {

test/parallel/test-buffer-tostring-range.js

@@ -82,3 +82,11 @@ assert.strictEqual(rangeBuffer.toString('ascii', 0, true), 'a');
 assert.strictEqual(rangeBuffer.toString({toString: function() {
   return 'ascii';
 }}), 'abc');
+
+// try toString() with 0 and null as the encoding
+assert.throws(() => {
+  rangeBuffer.toString(0, 1, 2);
+}, /^TypeError: Unknown encoding: 0$/);
+assert.throws(() => {
+  rangeBuffer.toString(null, 1, 2);
+}, /^TypeError: Unknown encoding: null$/);