buffer: allow .write() offset to be at buffer end

Do not throw if the offset passed to `buf.write()` points
to the end of the buffer.

Fixes: https://github.com/nodejs/node/issues/8127
PR-URL: https://github.com/nodejs/node/pull/8154
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

src/node_buffer.cc

@@ -714,7 +714,7 @@ void StringWrite(const FunctionCallbackInfo<Value>& args) {
   size_t max_length;
 
   CHECK_NOT_OOB(ParseArrayIndex(args[1], 0, &offset));
-  if (offset >= ts_obj_length)
+  if (offset > ts_obj_length)
     return env->ThrowRangeError("Offset is out of bounds");
 
   CHECK_NOT_OOB(ParseArrayIndex(args[2], ts_obj_length - offset, &max_length));

test/parallel/test-buffer-alloc.js

@@ -358,6 +358,12 @@ writeTest.write('e', 3, 'ascii');
 writeTest.write('j', 4, 'ascii');
 assert.equal(writeTest.toString(), 'nodejs');
 
+// Offset points to the end of the buffer
+// (see https://github.com/nodejs/node/issues/8127).
+assert.doesNotThrow(() => {
+  Buffer.alloc(1).write('', 1, 0);
+});
+
 // ASCII slice test
 {
   const asciiString = 'hello world';