Ensure lastMmapRef set to 0 if mmapRefs is empty

Signed-off-by: Carrie Edwards <edwrdscarrie@gmail.com>

tsdb/ooo_head_read.go

@@ -349,12 +349,17 @@ func NewOOOCompactionHead(ctx context.Context, head *Head) (*OOOCompactionHead,
 			continue
 		}
 
+		var lastMmapRef chunks.ChunkDiskMapperRef
 		mmapRefs := ms.mmapCurrentOOOHeadChunk(head.chunkDiskMapper)
 		if len(mmapRefs) == 0 && len(ms.ooo.oooMmappedChunks) > 0 {
 			// Nothing was m-mapped. So take the mmapRef from the existing slice if it exists.
 			mmapRefs = []chunks.ChunkDiskMapperRef{ms.ooo.oooMmappedChunks[len(ms.ooo.oooMmappedChunks)-1].ref}
 		}
-		lastMmapRef := mmapRefs[len(mmapRefs)-1]
+		if len(mmapRefs) == 0 {
+			lastMmapRef = 0
+		} else {
+			lastMmapRef = mmapRefs[len(mmapRefs)-1]
+		}
 		seq, off := lastMmapRef.Unpack()
 		if seq > lastSeq || (seq == lastSeq && off > lastOff) {
 			ch.lastMmapRef, lastSeq, lastOff = lastMmapRef, seq, off