Blender Git Loki

Fix T79357: VSE crash on prefetching when performing cuts

Caused by {rBa1a333a1e92e}

`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 @ISS is to just stop prefetching all together
before changing content of seqbase.

Maniphest Tasks: T79357

Differential Revision: https://developer.blender.org/D8421