btrfs: handle empty eb->folios in num_extent_folios()

num_extent_folios() unconditionally calls folio_order() on
eb->folios[0]. If that is NULL this will be a segfault. It is reasonable
for it to return 0 as the number of folios in the eb when the first
entry is NULL, so do that instead.

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