buffers: fix intermittent out of bounds error

The base64 decoder would intermittently throw an out-of-bounds exception when
the buffer in `buf.write('', 'base64')` was a zero-sized buffer located at the
end of the slab.

Fixes #2657.

src/node_buffer.cc

@@ -597,7 +597,7 @@ Handle<Value> Buffer::Base64Write(const Arguments &args) {
                                              : args[2]->Uint32Value();
   max_length = MIN(s.length(), MIN(buffer->length_ - offset, max_length));
 
-  if (offset >= buffer->length_) {
+  if (max_length && offset >= buffer->length_) {
     return ThrowException(Exception::TypeError(String::New(
             "Offset is out of bounds")));
   }

test/simple/test-buffer-regress-GH-2659.js

@@ -0,0 +1,30 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+// This is not a great test. It depends on a Node internal, namely the slab
+// size. Maybe we should expose that in some way. Then again, maybe not...
+for (var n = 1; n <= 8192; ++n) {
+  Buffer(n);
+  Buffer(0).write('', 'base64');
+}