In commit 8b9ad56e, we removed the code that could result
in our getting to sd_prealloc()'s out_with_err_set label with a
NULL blk pointer. That makes the NULL check in the error-handling
path unnecessary, and Coverity gripes about it (CID 1390636).
Delete the redundant check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>