mmc: block: fix free of uninitialized 'idata->buf'

Set 'idata->buf' to NULL so that it never gets returned without
initialization. This fixes a bug where mmc_blk_ioctl_cmd() would
free both 'idata' and 'idata->buf' but 'idata->buf' was returned
uninitialized.

Fixes: 1ff8950c ("mmc: block: change to use kmalloc when copy data from userspace")
Signed-off-by: Ville Viinikka <ville@tuxera.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>