Merge pull request #9119 from yudai/panic_defrag_fail

mvcc: check null before set FillPercent not to panic

mvcc/backend/backend.go

@@ -373,10 +373,10 @@ func defragdb(odb, tmpdb *bolt.DB, limit int) error {
 		}
 
 		tmpb, berr := tmptx.CreateBucketIfNotExists(next)
-		tmpb.FillPercent = 0.9 // for seq write in for each
 		if berr != nil {
 			return berr
 		}
+		tmpb.FillPercent = 0.9 // for seq write in for each
 
 		b.ForEach(func(k, v []byte) error {
 			count++