xfs: Fix possible null-pointer dereferences in xchk_da_btree_block_check_sibling()

In xchk_da_btree_block_check_sibling(), there is an if statement on
line 274 to check whether ds->state->altpath.blk[level].bp is NULL:
    if (ds->state->altpath.blk[level].bp)

When ds->state->altpath.blk[level].bp is NULL, it is used on line 281:
    xfs_trans_brelse(..., ds->state->altpath.blk[level].bp);
        struct xfs_buf_log_item *bip = bp->b_log_item;
        ASSERT(bp->b_transp == tp);

Thus, possible null-pointer dereferences may occur.

To fix these bugs, ds->state->altpath.blk[level].bp is checked before
being used.

These bugs are found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/scrub/dabtree.c

@@ -278,7 +278,11 @@ xchk_da_btree_block_check_sibling(
 	/* Compare upper level pointer to sibling pointer. */
 	if (ds->state->altpath.blk[level].blkno != sibling)
 		xchk_da_set_corrupt(ds, level);
-	xfs_trans_brelse(ds->dargs.trans, ds->state->altpath.blk[level].bp);
+	if (ds->state->altpath.blk[level].bp) {
+		xfs_trans_brelse(ds->dargs.trans,
+				ds->state->altpath.blk[level].bp);
+		ds->state->altpath.blk[level].bp = NULL;
+	}
 out:
 	return error;
 }