Raise InvalidURL if hosts starts with '.' as was reported at #5367.

Author: mondeja

requests/models.py

@@ -403,7 +403,7 @@ def prepare_url(self, url, params):
                 host = self._get_idna_encoded_host(host)
             except UnicodeError:
                 raise InvalidURL('URL has an invalid label.')
-        elif host.startswith(u'*'):
+        elif host.startswith((u'*', u'.')):
             raise InvalidURL('URL has an invalid label.')
 
         # Carefully reconstruct the network location

tests/test_requests.py

@@ -81,6 +81,8 @@ def test_entry_points(self):
             (InvalidSchema, 'localhost.localdomain:3128/'),
             (InvalidSchema, '10.122.1.1:3128/'),
             (InvalidURL, 'http://'),
+            (InvalidURL, 'http://*example.com'),
+            (InvalidURL, 'http://.example.com'),
         ))
     def test_invalid_url(self, exception, url):
         with pytest.raises(exception):