mvcc: check null before set FillPercent not to panic

Since CreateBucketIfNotExists() can return nil when it gets an error,
accessing FillPercent must be done after a nil check, not to cause
a panic.

mvcc/backend/backend.go

@@ -370,10 +370,10 @@ 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++