Interpreter doesn't validate "unused" bits terminal byte in varint

[binji]

See WebAssembly/wabt#799.

I belive the encoded value 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0xff 0x7e is not valid as a 64-bit signed varint, as the value it is decoded to, 9151314442816847872, should be encoded as 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0xff 0x00, where the top bits of the last byte are a sign-extension of the sign bit (the least-significant bit).

[binji]

cc @rossberg, am I misunderstanding the spec here? v8's implementation at least matches this behavior.

[rossberg]

No, this is indeed a bug in the interpreter, the shift for the mask it used was off by one. Fixed in #750, with a range of new binary tests for LEB encodings. PTAL