SDL client: crash when saving game with hidden term

[PowerWyrm]

How to reproduce:
- open the client with only term 0
- make term 1 visible
- save client (ok)
- open the client with both terms 0 and 1 visible
- make term 1 hidden
- save client (crash)

Crash occurs in sdl_FontFree(): TTF_CloseFont(font->sdl_font);
or sometimes simply in SDL_TTF.dll.

Feels like a double free on a font.

Adding a simple "font->sdl_font = NULL;" after "TTF_CloseFont(font->sdl_font);" in sdl_FontFree() fixes the bug.