xfs_metadump: ignore attr leaf with 0 entries

xfs_metadump: ignore attr leaf with 0 entries

Another in the ongoing saga of attribute leaves with zero
entries; in this case, if we try to metadump an inode with
a zero-entries attribute leaf, the zeroing code will go off
the rails and segfault at:

                memset(&entries[nentries], 0,
                       first_name - (char *)&entries[nentries]);

because first_name is null, and we try to memset a large
(negative) number.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>