storage: Increase persist watermark before calling append

The append call may reuse cds, and thus change its len.
(In practice, this wouldn't happen as cds should have len==cap.
Still, the previous order of lines was problematic.)

storage/local/series.go

@@ -464,9 +464,9 @@ func (s *memorySeries) preloadChunksForRange(
 				fp, s.chunkDescsOffset, len(cds),
 			)
 		}
+		s.persistWatermark += len(cds)
 		s.chunkDescs = append(cds, s.chunkDescs...)
 		s.chunkDescsOffset = 0
-		s.persistWatermark += len(cds)
 		if len(s.chunkDescs) > 0 {
 			firstChunkDescTime = s.chunkDescs[0].FirstTime()
 		}