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>