fs: change statSync to accessSync in realpathSync

fs.statSync() creates and returns a heavy-weight fs.Stat object whereas
fs.accessSync() simply returns nothing.  The return value is ignored,
the call is for its side effect of throwing an ELOOP error in case of
cyclic symbolic links.

PR-URL: nodejs#4575
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/fs.js

@@ -1550,7 +1550,7 @@ fs.realpathSync = function realpathSync(p, cache) {
         }
       }
       if (linkTarget === null) {
-        fs.statSync(base);
+        fs.accessSync(base, fs.F_OK);  // Throws ELOOP on cyclic links.
         linkTarget = fs.readlinkSync(base);
       }
       resolvedLink = pathModule.resolve(previous, linkTarget);