EventIndex: Handle invalid m.room.redaction events correctly.
EventIndex: Handle invalid m.room.redaction events correctly.

Clients might send invalid redaction events, such events will not have a
valid redacts field containing the event id of the associated event that
was redacted.

Skip such events instead of ending up looping over the checkpoint
forever.

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk