Fix T85587: Crash on selecting multiple frames

Blender would crash when selecting multiple keyframes while multiframe edit was
active. This was due to the active frame being NULL in some instances.

The fix checks if the active frame is not NULL.

Reviewed By: antoniov

Maniphest Tasks: T85587

Differential Revision: https://developer.blender.org/D10421

source/blender/blenkernel/intern/gpencil.c

@@ -2605,7 +2605,7 @@ void BKE_gpencil_visible_stroke_iter(ViewLayer *view_layer,
       sta_gpf = end_gpf = NULL;
       /* Check the whole range and tag the editable frames. */
       LISTBASE_FOREACH (bGPDframe *, gpf, &gpl->frames) {
-        if (gpf == act_gpf || (gpf->flag & GP_FRAME_SELECT)) {
+        if (act_gpf != NULL && (gpf == act_gpf || (gpf->flag & GP_FRAME_SELECT))) {
           gpf->runtime.onion_id = 0;
           if (do_onion) {
             if (gpf->framenum < act_gpf->framenum) {