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: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_bmap.c

@@ -4014,7 +4014,8 @@ xfs_bmapi_allocate(
 	if (bma->wasdel) {
 		bma->length = (xfs_extlen_t)bma->got.br_blockcount;
 		bma->offset = bma->got.br_startoff;
-		xfs_iext_peek_prev_extent(ifp, &bma->icur, &bma->prev);
+		if (!xfs_iext_peek_prev_extent(ifp, &bma->icur, &bma->prev))
+			bma->prev.br_startoff = NULLFILEOFF;
 	} else {
 		bma->length = XFS_FILBLKS_MIN(bma->length, MAXEXTLEN);
 		if (!bma->eof)