xfs: fix btree scrub deref check

The btree scrubber has some custom code to retrieve and check a btree
block via xfs_btree_lookup_get_block.  This function will either return
an error code (verifiers failed) or a *pblock will be untouched (bad
pointer).  Since we previously set *pblock to NULL, we need to check
*pblock, not pblock, to trigger the early bailout.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/scrub/btree.c

@@ -335,7 +335,7 @@ xfs_scrub_btree_get_block(
 
 	error = xfs_btree_lookup_get_block(bs->cur, level, pp, pblock);
 	if (!xfs_scrub_btree_process_error(bs->sc, bs->cur, level, &error) ||
-	    !pblock)
+	    !*pblock)
 		return error;
 
 	xfs_btree_get_block(bs->cur, level, pbp);