fs: avoid multiple conversions to string

nullCheck() implicitly converts the argument to string when checking
the value, so this commit avoids any unnecessary additional (Buffer)
conversions to string.

PR-URL: https://github.com/nodejs/node/pull/10789
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/fs.js

@@ -1503,21 +1503,21 @@ function encodeRealpathResult(result, options) {
 fs.realpathSync = function realpathSync(p, options) {
   options = getOptions(options, {});
   handleError((p = getPathFromURL(p)));
+  if (typeof p !== 'string')
+    p += '';
   nullCheck(p);
-
-  p = p.toString('utf8');
   p = pathModule.resolve(p);
 
-  const seenLinks = {};
-  const knownHard = {};
   const cache = options[internalFS.realpathCacheKey];
-  const original = p;
-
   const maybeCachedResult = cache && cache.get(p);
   if (maybeCachedResult) {
     return maybeCachedResult;
   }
 
+  const seenLinks = {};
+  const knownHard = {};
+  const original = p;
+
   // current character position in p
   var pos;
   // the partial path so far, including a trailing slash if any
@@ -1614,10 +1614,10 @@ fs.realpath = function realpath(p, options, callback) {
   options = getOptions(options, {});
   if (handleError((p = getPathFromURL(p)), callback))
     return;
+  if (typeof p !== 'string')
+    p += '';
   if (!nullCheck(p, callback))
     return;
-
-  p = p.toString('utf8');
   p = pathModule.resolve(p);
 
   const seenLinks = {};