Fix crash in membership updater

[neilalexander]

While the behaviour of #1748 was definitely not right, it turns out it was actually preventing us from dereferencing a nil pointer which would cause a crash.

This behaviour is probably better — comments inline to explain why, since it’s especially fiddly.

[f12systems]

ran this on my machine and got a different 'crash' that seems to indicate the earlier issue
left the dB in a locked state

Specifically
Signalling other goroutines waiting for this goroutine to generate the thumbnail. Height=480 MediaID=QiYNbjnWfZTiNYOaXhQGRNDW Origin=matrix.org ResizeMethod=scale Width=640 req.id=04NKqGdcNXgV req.method=GET req.path=/_matrix/media/r0/thumbnail/matrix.org/QiYNbjnWfZTiNYOaXhQGRNDW
panic: the ContinualConsumer in "syncapi/clientapi" failed to SetPartitionOffset: database is locked

goroutine 72 [running]:
github.com/matrix-org/dendrite/internal.(*ContinualConsumer).consumePartition(0xc0003aa300, 0x558211175340, 0xc000372400)
github.com/matrix-org/dendrite/internal/consumers.go:129 +0x2ed
created by github.com/matrix-org/dendrite/internal.(*ContinualConsumer).StartOffsets
github.com/matrix-org/dendrite/internal/consumers.go:107 +0x545

[neilalexander]

This has fixed crashes on d.m.org and d.n.dev so merging.