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: #4158
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Trevor Norris <[email protected]>
Reviewed-By: Minwoo Jung <[email protected]>

lib/buffer.js

@@ -40,7 +40,7 @@ function alignPool() {
 }
 
 
-function Buffer(arg) {
+function Buffer(arg, encoding) {
   // Common case.
   if (typeof arg === 'number') {
     // If less than zero, or NaN.
@@ -51,7 +51,7 @@ function Buffer(arg) {
 
   // Slightly less common case.
   if (typeof arg === 'string') {
-    return fromString(arg, arguments[1]);
+    return fromString(arg, encoding);
   }
 
   // Unusual.