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

source/blender/editors/space_sequencer/sequencer_edit.c

@@ -2337,6 +2337,8 @@ static int sequencer_split_exec(bContext *C, wmOperator *op)
   split_side = RNA_enum_get(op->ptr, "side");
   ignore_selection = RNA_boolean_get(op->ptr, "ignore_selection");
 
+  BKE_sequencer_prefetch_stop(scene);
+
   if (split_hard == SEQ_SPLIT_HARD) {
     changed = split_seq_list(bmain,
                              scene,