xfs_repair: treat zero da btree pointers as corruption

xfs_repair: treat zero da btree pointers as corruption

If a da btree pointer is zero (i.e. the beginning of the fork) report
this as a corrupt tree to the caller instead of telling it that
everything is good.  Fixes assertion errors when fuzzing
nbtree[0].before to zero in xfs/394.

[sandeen: tweak comment above change for clarity]

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