Implement workaround for login error.

Signed-off-by: Robert Jacob <xperimental@solidproject.de>

changelog.d/4486.bugfix

@@ -0,0 +1 @@
+Workaround for login error when using both LDAP and internal authentication.

synapse/util/logcontext.py

@@ -285,7 +285,10 @@ class LoggingContext(object):
         self.alive = False
 
         # if we have a parent, pass our CPU usage stats on
-        if self.parent_context is not None:
+        if (
+            self.parent_context is not None
+            and hasattr(self.parent_context, '_resource_usage')
+        ):
             self.parent_context._resource_usage += self._resource_usage
 
             # reset them in case we get entered again