Add handling malformed inputs to function `read_varint` and test cases.
```
# 9 byte truncated to 8
read_varint(&[0x81, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80])
before -> panic index out of bounds: the len is 8 but the index is 8
after -> LimboError
# bits set without end
read_varint(&[0x80; 9])
before -> Ok((128, 9))
after -> LimboError
```
Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Closes#2904
382a1e14ca