Fix crash when dragging plugins from sidebar

gtk2_ardour/processor_box.cc

@@ -2155,6 +2155,10 @@ not match the configuration of this track.");
 bool
 ProcessorBox::drag_refuse (DnDVBox<ProcessorEntry>* source, ProcessorEntry*)
 {
+	if (!source) {
+		/* handle drag from sidebar */
+		return false;
+	}
 	ProcessorBox* other = reinterpret_cast<ProcessorBox*> (source->get_data ("processorbox"));
 	return (other && other->_route == _route);
 }

gtk2_ardour/region_editor.cc

@@ -937,6 +937,9 @@ RegionEditor::RegionFxBox::delete_dragged_plugins (Region::RegionFxList const& f
 bool
 RegionEditor::RegionFxBox::drag_refuse (Gtkmm2ext::DnDVBox<RegionFxEntry>* source, RegionFxEntry*)
 {
+	if (!source) {
+		return false;
+	}
 	RegionFxBox* other = reinterpret_cast<RegionFxBox*> (source->get_data ("regionfxbox"));
 	return (other && other->_region == _region);
 }