aboot: Check mmc device pointer for NULL before using it
aboot: Check mmc device pointer for NULL before using it

jsr-d10/android_bootable_bootloader_lk,jsr-d10/android_bootable_bootloader_lk,jsr-d10/android_bootable_bootloader_lk,jsr-d10/android_bootable_bootloader_lk,jsr-d10/android_bootable_bootloader_lk