buffer: fix Buffer::Copy regression from 00b4b7bb

If the end argument is omitted or not a number, make it default to
the end of the buffer, not zero.

Ideally, it should not matter what it defaults to because the JS shim
in lib/buffer.js should handle that but there are still several places
in node.js core that secrete SlowBuffers, hence Buffer::Copy() gets
called without going through Buffer.prototype.copy() first.