gamestate_inspector: add a ctor initialization for lua_button ptr

This was a minor oversight by me, issue raised by Coverity.

src/gui/dialogs/gamestate_inspector.cpp

@@ -128,6 +128,7 @@ class tgamestate_inspector::model
 		, inspect()
 		, inspector_name()
 		, copy_button()
+		, lua_button()
 	{
 		name = cfg["name"].str();
 	}