Fix off-by-one error that could panic.

tsdb/engine/tsm1/encoding.go

@@ -625,13 +625,13 @@ func DecodeStringBlock(block []byte, tdec *TimeDecoder, vdec *StringDecoder, a *
 func packBlock(buf []byte, typ byte, ts []byte, values []byte) []byte {
 	// We encode the length of the timestamp block using a variable byte encoding.
 	// This allows small byte slices to take up 1 byte while larger ones use 2 or more.
-	sz := 1 + 10 + len(ts) + len(values)
+	sz := 1 + binary.MaxVarintLen64 + len(ts) + len(values)
 	if cap(buf) < sz {
 		buf = make([]byte, sz)
 	}
 	b := buf[:sz]
 	b[0] = typ
-	i := binary.PutUvarint(b[1:10], uint64(len(ts)))
+	i := binary.PutUvarint(b[1:1+binary.MaxVarintLen64], uint64(len(ts)))
 	i += 1
 
 	// block is <len timestamp bytes>, <ts bytes>, <value bytes>