Fix Buffer.toString() on 0-length slices.

- Buffer.toString('ascii', 0, 0) incorrectly returns the entire contents
  of the buffer. Fix this.
- Provide similar behavior to Buffer.write() and Buffer.copy() when
  dealing with 0-length in valid and invalid byte ranges.

lib/buffer.js

@@ -24,7 +24,12 @@ Buffer.prototype.inspect = function () {
 Buffer.prototype.toString = function (encoding, start, stop) {
   encoding = (encoding || 'utf8').toLowerCase();
   if (!start) start = 0;
-  if (!stop) stop = this.length;
+  if (stop === undefined) stop = this.length;
+
+  // Fastpath empty strings
+  if (stop === start) {
+    return '';
+  }
 
   switch (encoding) {
     case 'utf8':

test/simple/test-buffer.js

@@ -131,6 +131,12 @@ b.copy(new Buffer(1), 1, 1, 1);
 // try to copy 0 bytes from past the end of the source buffer
 b.copy(new Buffer(1), 0, 2048, 2048);
 
+// try to toString() a 0-length slice of a buffer, both within and without the
+// valid buffer range
+assert.equal(new Buffer('abc').toString('ascii', 0, 0), '');
+assert.equal(new Buffer('abc').toString('ascii', -100, -100), '');
+assert.equal(new Buffer('abc').toString('ascii', 100, 100), '');
+
 var asciiString = "hello world";
 var offset = 100;
 for (var j = 0; j < 500; j++) {