Fix timeline marker click-drag arbitrary deselection

The logic to cycle selected markers wasn't cycling back to the beginning
of the list.

The marker after the selected marker at the cursor frame was also used
to check if a selection existed, causing dragging to transform all
selected markers to de-select all when when dragging the last marker.

source/blender/editors/animation/anim_markers.c

@@ -1146,7 +1146,7 @@ static int select_timeline_marker_frame(ListBase *markers,
   for (marker = markers->first; marker; marker = marker->next) {
     if (marker->frame == frame) {
       if (marker->flag & SELECT) {
-        marker_selected = marker->next;
+        marker_selected = marker->next ? marker->next : markers->first;
         break;
       }
     }