btrfs: replace BUG_ON with ASSERT in btrfs_read_node_slot

In btrfs_read_node_slot() we have a BUG_ON() that can be converted to an
ASSERT(), it's from an extent buffer and the level is validated at the
time it's read from disk.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>