Handle F_OK case in fs__access()

It turns out that `F_OK` is supposed to only check for path existence,
and because it is `0x00`, it ends up confusing the Windows APIs below
where it looks like we're asking for the null set of permissions.

src/win/fs.c

@@ -2124,7 +2124,18 @@ static void fs__access(uv_fs_t* req) {
     SET_REQ_WIN32_ERROR(req, GetLastError());
     return;
   }
-  DWORD sdLen = 0, err = 0, tokenAccess = 0, executeAccessRights = 0,
+
+  /*
+   * If the calling code is asking for `F_OK`, it merely cares about
+   * path existence, which we have already determined, so early-exit
+   * before we start checking individual bits in `mode`.
+   */
+  if (req->fs.info.mode == F_OK) {
+    SET_REQ_SUCCESS(req);
+    return;
+  }
+
+  DWORD sdLen = 0, err = 0, tokenAccess = 0, desiredAccess = 0,
         grantedAccess = 0, privilegesLen = 0;
   SECURITY_INFORMATION si = (SECURITY_INFORMATION)NULL;
   PSECURITY_DESCRIPTOR sd = NULL;
@@ -2185,26 +2196,26 @@ static void fs__access(uv_fs_t* req) {
     * Next, construct a mapping from generic access rights to
     * the more specific access rights that AccessCheck expects.
     */
-  executeAccessRights = 0;
+  desiredAccess = 0;
   if (req->fs.info.mode & R_OK) {
-    executeAccessRights |= FILE_GENERIC_READ;
+    desiredAccess |= FILE_GENERIC_READ;
     mapping.GenericRead = FILE_GENERIC_READ;
   }
   if (req->fs.info.mode & W_OK) {
-    executeAccessRights |= FILE_GENERIC_WRITE;
+    desiredAccess |= FILE_GENERIC_WRITE;
     mapping.GenericWrite = FILE_GENERIC_WRITE;
   }
   if (req->fs.info.mode & X_OK) {
-    executeAccessRights |= FILE_GENERIC_EXECUTE;
+    desiredAccess |= FILE_GENERIC_EXECUTE;
     mapping.GenericExecute = FILE_GENERIC_EXECUTE;
   }
-  MapGenericMask(&executeAccessRights, &mapping);
+  MapGenericMask(&desiredAccess, &mapping);
 
   privilegesLen = sizeof(privileges);
   result = FALSE;
   if (AccessCheck(sd,
                   hImpersonatedToken,
-                  executeAccessRights,
+                  desiredAccess,
                   &mapping,
                   &privileges,
                   &privilegesLen,