publisher: reset substitution sources early on (#6833)

In case of access to an unknown host try_publish() would bail out early with a
TraversalError() (404), which would access the publisher known substitution
variables to render the error page.  That list was not reset yet from the
previous call and could thus result in inappropriate variables being created.

wcs/qommon/publisher.py

@@ -511,6 +511,8 @@ class QommonPublisher(Publisher):
         return False
 
     def try_publish(self, request):
+        self.substitutions.reset()
+
         try:
             self.set_app_dir(request)
         except ImmediateRedirectException, e:
@@ -577,7 +579,6 @@ class QommonPublisher(Publisher):
 
         request.language = self.get_site_language()
         self.install_lang(request.language)
-        self.substitutions.reset()
         self.substitutions.feed(self)
         self.substitutions.feed(request)
         for extra_source in self.extra_sources: