block/cow: Avoid use of uninitialized cow_bs in error path

Commit 25814e8987 introduced an error-exit code path which does
a "goto exit" before the cow_bs variable is initialized, meaning
we would call bdrv_unref() on an uninitialized variable and
likely segfault. Fix this by moving the NULL-initialization
to the top of the function and making the exit code path handle
the case where it is NULL.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
