crush: ensure bucket id is valid before indexing buckets array

We were indexing the buckets array without verifying the index was
within the [0,max_buckets) range.  This could happen because
a multistep rule does not have enough buckets and has CRUSH_ITEM_NONE
for an intermediate result, which would feed in CRUSH_ITEM_NONE and
make us crash.

Reflects ceph.git commit 976a24a326da8931e689ee22fce35feab5b67b76.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>