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>

fs/btrfs/extent_io.h

@@ -298,6 +298,8 @@ static inline int __pure num_extent_pages(const struct extent_buffer *eb)
  */
 static inline int __pure num_extent_folios(const struct extent_buffer *eb)
 {
+	if (!eb->folios[0])
+		return 0;
 	if (folio_order(eb->folios[0]))
 		return 1;
 	return num_extent_pages(eb);