crush: assume weight_set != null imples weight_set_size > 0

Reflects ceph.git commit 5e8fa3e06b68fae1582c9230a3a8d1abc6146286.

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