fix segfault with new sidebar styles (#5416)

Stack trace:
```
0  0x08147a1d in curses_init () at src/sdltiles.cpp:1070
1  0x08469693 in initscr () at src/cursesport.cpp:25
2  0x081146e4 in main (argc=0, argv=0xbfffebe8) at src/main.cpp:77
```
At that point the global game instance has not yet been created which makes any access to `game::narrow_sidebar` an error.