Cast a coroutine into a Deferred in the federation base (#6996)

Properly convert a coroutine into a Deferred in federation_base to fix an error when joining a room.

changelog.d/6996.bugfix

@@ -0,0 +1 @@
+Fix bug which caused an error when joining a room, with `'coroutine' object has no attribute 'event_id'`.

synapse/federation/federation_base.py

@@ -96,13 +96,15 @@ class FederationBase(object):
 
             if not res and pdu.origin != origin:
                 try:
-                    res = yield self.get_pdu(
+                    res = yield defer.ensureDeferred(
+                        self.get_pdu(
                             destinations=[pdu.origin],
                             event_id=pdu.event_id,
                             room_version=room_version,
                             outlier=outlier,
                             timeout=10000,
                         )
+                    )
                 except SynapseError:
                     pass