buffer: Prevent Buffer constructor deopt

The Buffer constructor will generally get inlined, but any call to the Buffer
constructor for a string without encoding will cause an eager deoptimization
of any function that inlined the Buffer constructor. This is due to a an
out-of-bounds read on `arguments[1]`. This change prevents that deopt.

PR-URL: https://github.com/nodejs/node/pull/4158


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>