Fix selected item in dropdown for conditional to test slot occupancy state

src/tracerestrict_gui.cpp

@@ -1364,8 +1364,7 @@ public:
 				switch (GetTraceRestrictTypeProperties(item).value_type) {
 					case TRVT_SLOT_INDEX_INT: {
 						int selected;
-						DropDownList *dlist = GetSlotDropDownList(this->GetOwner(),
-								*(TraceRestrictProgram::InstructionAt(this->GetProgram()->items, this->selected_instruction - 1) + 1), selected);
+						DropDownList *dlist = GetSlotDropDownList(this->GetOwner(), GetTraceRestrictValue(item), selected);
 						if (dlist != NULL) ShowDropDownList(this, dlist, selected, TR_WIDGET_LEFT_AUX_DROPDOWN);
 						break;
 					}