assert: indicate if exception message is generated

AssertionError.generatedMessage is now true when
AssertionError.message was generated from expected and actual

Fixes #5836, #6206
buffer: check data is not null

Because it's possible for the data within a Buffer instance to be
altered after instantiation, or in case a user attempts to do something
like the following:

Buffer.prototype.fill.call({}, 10, 0, 10);

It doesn't result in a segfault.
buffer: add buf.toArrayBuffer() API
lint: fix a cpplint error

Added a NOLINT so that cpplint won't complain about some code.