Fix errors in sequencer channel headers

Failing constraint test uncovered crashes on NULL dereference and
missing channels initialization in `SEQ_editing_ensure()`.

source/blender/sequencer/intern/sequencer.c

@@ -245,6 +245,8 @@ Editing *SEQ_editing_ensure(Scene *scene)
     ed->cache = NULL;
     ed->cache_flag = SEQ_CACHE_STORE_FINAL_OUT;
     ed->cache_flag |= SEQ_CACHE_STORE_RAW;
+    ed->displayed_channels = &ed->channels;
+    SEQ_channels_ensure(ed->displayed_channels);
   }
 
   return scene->ed;