xfs: check for null dquot in xfs_quota_calc_throttle()

Coverity spotted this.

Granted, we *just* checked xfs_inod_dquot() in the caller (by
calling xfs_quota_need_throttle). However, this is the only place we
don't check the return value but the check is cheap and future-proof
so add it.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_iomap.c

@@ -404,8 +404,8 @@ xfs_quota_calc_throttle(
 	int shift = 0;
 	struct xfs_dquot *dq = xfs_inode_dquot(ip, type);
 
-	/* over hi wmark, squash the prealloc completely */
-	if (dq->q_res_bcount >= dq->q_prealloc_hi_wmark) {
+	/* no dq, or over hi wmark, squash the prealloc completely */
+	if (!dq || dq->q_res_bcount >= dq->q_prealloc_hi_wmark) {
 		*qblocks = 0;
 		*qfreesp = 0;
 		return;