6842 Fix empty xattr dir causing lockup
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>

*** 573,583 ****
          uint64_t idx, blk;
          int err;
  
          ASSERT(zap->zap_dbuf == NULL ||
              zap_f_phys(zap) == zap->zap_dbuf->db_data);
!         ASSERT3U(zap_f_phys(zap)->zap_magic, ==, ZAP_MAGIC);
          idx = ZAP_HASH_IDX(h, zap_f_phys(zap)->zap_ptrtbl.zt_shift);
          err = zap_idx_to_blk(zap, idx, &blk);
          if (err != 0)
                  return (err);
          err = zap_get_leaf_byblk(zap, blk, tx, lt, lp);
--- 573,590 ----
          uint64_t idx, blk;
          int err;
  
          ASSERT(zap->zap_dbuf == NULL ||
              zap_f_phys(zap) == zap->zap_dbuf->db_data);
! 
!         /* Reality check for corrupt zap objects (leaf or header). */
!         if ((zap_f_phys(zap)->zap_block_type != ZBT_LEAF &&
!             zap_f_phys(zap)->zap_block_type != ZBT_HEADER) ||
!             zap_f_phys(zap)->zap_magic != ZAP_MAGIC) {
!                 return (SET_ERROR(EIO));
!         }
! 
          idx = ZAP_HASH_IDX(h, zap_f_phys(zap)->zap_ptrtbl.zt_shift);
          err = zap_idx_to_blk(zap, idx, &blk);
          if (err != 0)
                  return (err);
          err = zap_get_leaf_byblk(zap, blk, tx, lt, lp);