GBA SIO: Fix unitialized memory issue
GBA SIO: Fix unitialized memory issue

mgba-emu/mgba,libretro/mgba,libretro/mgba,Iniquitatis/mgba,Iniquitatis/mgba,Iniquitatis/mgba,libretro/mgba,Iniquitatis/mgba,mgba-emu/mgba,libretro/mgba,mgba-emu/mgba,libretro/mgba,mgba-emu/mgba