message: Add test showing UB in leb128s_decode<int8_t>
diff: @@ -163,6 +163,7 @@ TEST(leb128, decode_s8)
  {{0x81, 0x00}, 1}, // 1 with leading zero
  {{0xff, 0x01}, -1},
  {{0xfe, 0x01}, -2},
  + {{0x40}, -64},
  };
  // clang-format on
  