only try to LL members after a non-gappy incr sync if the timeline has events

changelog.d/3936.bugfix

@@ -0,0 +1 @@
+Fix out-of-bounds error when LLing yourself

synapse/handlers/sync.py

@@ -794,7 +794,7 @@ def compute_state_delta(self, room_id, batch, sync_config, since_token, now_toke
             else:
                 state_ids = {}
                 if lazy_load_members:
-                    if types:
+                    if types and batch.events:
                         # We're returning an incremental sync, with no
                         # "gap" since the previous sync, so normally there would be
                         # no state to return.