[niconico] Fix ignored --netrc flag

See issue #3753

youtube_dl/extractor/niconico.py

@@ -39,18 +39,17 @@ class NiconicoIE(InfoExtractor):
 
     _VALID_URL = r'https?://(?:www\.|secure\.)?nicovideo\.jp/watch/((?:[a-z]{2})?[0-9]+)'
     _NETRC_MACHINE = 'niconico'
-    # Determine whether the downloader uses authentication to download video
-    _AUTHENTICATE = False
+    # Determine whether the downloader used authentication to download video
+    _AUTHENTICATED = False
 
     def _real_initialize(self):
-        if self._downloader.params.get('username', None) is not None:
-            self._AUTHENTICATE = True
-
-        if self._AUTHENTICATE:
         self._login()
 
     def _login(self):
         (username, password) = self._get_login_info()
+        # No authentication to be performed
+        if not username:
+            return True
 
         # Log in
         login_form_strs = {
@@ -68,6 +67,8 @@ class NiconicoIE(InfoExtractor):
         if re.search(r'(?i)<h1 class="mb8p4">Log in error</h1>', login_results) is not None:
             self._downloader.report_warning('unable to log in: bad username or password')
             return False
+        # Successful login
+        self._AUTHENTICATED = True
         return True
 
     def _real_extract(self, url):
@@ -82,7 +83,7 @@ class NiconicoIE(InfoExtractor):
             'http://ext.nicovideo.jp/api/getthumbinfo/' + video_id, video_id,
             note='Downloading video info page')
 
-        if self._AUTHENTICATE:
+        if self._AUTHENTICATED:
             # Get flv info
             flv_info_webpage = self._download_webpage(
                 'http://flapi.nicovideo.jp/api/getflv?v=' + video_id,