fix: Don't panic on missing SSH in sliding sync

src/api/client/sync/v5.rs

@@ -868,8 +868,11 @@ where
 					.ignore_err();
 			}
 
-			let (_, pdu_at_last_sync_end) = pdus_rev.next().await?;
+			let (count, pdu_at_last_sync_end) = pdus_rev.next().await?;
 
+			if matches!(count, PduCount::Backfilled(_)) {
+				None
+			} else {
 				Some(
 					services
 						.rooms
@@ -879,6 +882,7 @@ where
 						.expect("pdu should have a shortstatehash"),
 				)
 			}
+		}
 		.await;
 
 		let encrypted_room = services