hw/arm/orangepi: check for potential NULL pointer when calling blk_is_available

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>

hw/arm/orangepi.c

@@ -104,7 +104,7 @@ static void orangepi_init(MachineState *machine)
                                 machine->ram);
 
     /* Load target kernel or start using BootROM */
-    if (!machine->kernel_filename && blk_is_available(blk)) {
+    if (!machine->kernel_filename && blk && blk_is_available(blk)) {
         /* Use Boot ROM to copy data from SD card to SRAM */
         allwinner_h3_bootrom_setup(h3, blk);
     }