xfs_repair: check num before bplist[num]

smatch complained about checking an array index before indexing the
array, so fix that.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/repair/prefetch.c b/repair/prefetch.c
index de36c5f..22efd54 100644
--- a/repair/prefetch.c
+++ b/repair/prefetch.c
@@ -494,7 +494,7 @@
 						args->last_bno_read, &fsbno);
 			max_fsbno = fsbno + pf_max_fsbs;
 		}
-		while (bplist[num] && num < MAX_BUFS && fsbno < max_fsbno) {
+		while (num < MAX_BUFS && bplist[num] && fsbno < max_fsbno) {
 			/*
 			 * Discontiguous buffers need special handling, so stop
 			 * gathering new buffers and process the list and this