xfs: null out bma->prev if no previous extent

Coverity complains that we don't check the return value of
xfs_iext_peek_prev_extent like we do nearly all of the time.  If there
is no previous extent then just null out bma->prev like we do elsewhere
in the bmap code.

Coverity-id: 1424057
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>