The Orange Pi PC initialization function needs to verify that the SD card
block backend is usable before calling the Boot ROM setup routine. When
calling blk_is_available() the input parameter should not be NULL.
This commit ensures that blk_is_available is only called with non-NULL input.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200322205439.15231-1-nieklinnenbank@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>