Blender Git Loki

Fix (unreported): Crash on accessing active sequence in select groupped operator

This patch moves the NULL check of `actseq` to the correct position, which should happen
before the `channel` is assigned. Otherwise an attempt to call the `sequencer_select_grouped_exec`,
when there is no active sequence and `use_active_channel` set to true, results in a crash.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D7170