emulator: don't crash if SDMMC device is accessed past end of volume or no image specified
emulator: don't crash if SDMMC device is accessed past end of volume or no image specified

Time out on init if there is no volume, return 0xff past end of volume

FulcronZ/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,FulcronZ/NyuziProcessor,FulcronZ/NyuziProcessor