Blender Git Loki

Fix T76033: VSE crash with prefetch, disk cache and meta strips

`BKE_sequencer_prefetch_get_original_sequence()` didn't look in metas
and returned NULL. This caused crash in disk cache that was trying to
read seq->name.

Add function that will look in meta strips recursively and condition
that seq must not be NULL.

Reviewed By: brecht

Maniphest Tasks: T76033

Differential Revision: https://developer.blender.org/D7597