Fix #7974: Crash when CTRL+click to show a vehicle group that is collapsed

src/group_gui.cpp

@@ -1017,6 +1017,7 @@ public:
 				}
 				this->groups.ForceRebuild();
 				this->BuildGroupList(this->owner);
+				this->group_sb->SetCount((uint)this->groups.size());
 				id_g = find_index(this->groups, g);
 			}
 			this->group_sb->ScrollTowards(id_g);