Fix T41961: Crash by Copy n Paste splines to new mask without layer

It makes much more sense to ensure layer before pasteing, makes roto
as easy as it has never been before!

source/blender/editors/mask/mask_ops.c

@@ -2331,6 +2331,10 @@ static int paste_splines_exec(bContext *C, wmOperator *UNUSED(op))
 	Mask *mask = CTX_data_edit_mask(C);
 	MaskLayer *mask_layer = BKE_mask_layer_active(mask);
 
+	if (mask_layer == NULL) {
+		mask_layer = BKE_mask_layer_new(mask, "");
+	}
+
 	BKE_mask_clipboard_paste_to_layer(CTX_data_main(C), mask_layer);
 
 	/* TODO: only update edited splines */