Libretro: Fix a crash if no core is loaded
Libretro: Fix a crash if no core is loaded

sergiobenrocha2/mgba,libretro/mgba,Anty-Lemon/mgba,iracigt/mgba,Anty-Lemon/mgba,fr500/mgba,sergiobenrocha2/mgba,jeremyherbert/mgba,Iniquitatis/mgba,fr500/mgba,mgba-emu/mgba,Touched/mgba,Anty-Lemon/mgba,mgba-emu/mgba,Touched/mgba,MerryMage/mgba,Touched/mgba,MerryMage/mgba,iracigt/mgba,mgba-emu/mgba,iracigt/mgba,jeremyherbert/mgba,sergiobenrocha2/mgba,libretro/mgba,libretro/mgba,sergiobenrocha2/mgba,fr500/mgba,jeremyherbert/mgba,libretro/mgba,iracigt/mgba,Anty-Lemon/mgba,fr500/mgba,libretro/mgba,Iniquitatis/mgba,Iniquitatis/mgba,sergiobenrocha2/mgba,Iniquitatis/mgba,mgba-emu/mgba,jeremyherbert/mgba,MerryMage/mgba