Fix #8332: aborting group drag&drop could cause crashes with vehicle drag&drop (#8511)

The selected group was not reset when drag&drop was aborted. When
after that vehicle drag&drop was successful, group drag&drop code
was still executed, causing weird behaviour or even crashes.

src/group_gui.cpp

@@ -981,6 +981,7 @@ public:
 		/* abort drag & drop */
 		this->vehicle_sel = INVALID_VEHICLE;
 		this->DirtyHighlightedGroupWidget();
+		this->group_sel = INVALID_GROUP;
 		this->group_over = INVALID_GROUP;
 		this->SetWidgetDirty(WID_GL_LIST_VEHICLE);
 	}