Caused by rBa1a333a1e92e: Fix T76774: Crash on prefetching sequences from another scene.

BKE_sequencer_prefetch_get_original_sequence cant get a sequence in
BKE_sequencer_cache_put.

rBa1a333a1e92e moved multiple NULL checks (including the one checking
seq) above BKE_sequencer_prefetch_get_original_sequence (this should
never return NULL really, but this is for another fix).

So solution suggested by @Richard Antalik (ISS) is to just stop prefetching all together
before changing content of seqbase.