btrfs: deal with missing root in sample_block_group_extent_item()

btrfs: deal with missing root in sample_block_group_extent_item()

In case the root does not exists, which is unexpected, btrfs_extent_root()
returns NULL, but we ignore that and so if it happens we can trigger a
NULL pointer dereference later. So verify if we found the root and log an
error message in case it's missing.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>