hgweb: log headers only if headers were successfully parsed

The headers attribute is not initialized in certain error situations
(e.g. http 400 bad request). Check for self.headers before we attempt
to access it.

mercurial/hgweb/server.py

@@ -60,7 +60,10 b' class _httprequesthandler(BaseHTTPServer'
         self._log_any(self.server.accesslog, format, *args)
 
     def log_request(self, code='-', size='-'):
-        xheaders = [h for h in self.headers.items() if h[0].startswith('x-')]
+        xheaders = []
+        if util.safehasattr(self, 'headers'):
+            xheaders = [h for h in self.headers.items()
+                        if h[0].startswith('x-')]
         self.log_message('"%s" %s %s%s',
                          self.requestline, str(code), str(size),
                          ''.join([' %s:%s' % h for h in sorted(xheaders)]))