sd.c: Handle NULL block backend in sd_get_inserted()

The sd.c SD card emulation code can be in a state where the
SDState BlockBackend pointer is NULL; this is treated as
"card not present". Add a missing check to sd_get_inserted()
so that we don't segfault in this situation.

(This could be provoked by the guest writing to the SDHCI
register to do a reset on a xilinx-zynq-a9 board; it will
also happen at startup when sdhci implements its DeviceClass
reset method.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1456493044-10025-2-git-send-email-peter.maydell@linaro.org
