Crash on null dereference in `SEQ_timeline_boundbox()`. This function was generalized in rB9e4a4c2e996c to work on arbitrary `seqbase`. Fixed by refactoring `SEQ_timeline_boundbox()` functions to return default sane values if `seqbase` is `NULL` Reviewed By: HooglyBoogly Differential Revision: https://developer.blender.org/D9878