Fix mload8 emulation in signatureSplit
Fix mload8 emulation in signatureSplit

Ran across this while reviewing the netting code.

To grab the lower 8 bits should be `(x & 0xff)`, not `(x & 0x01)`. For
`v` in `[0, 1]` (old versions of geth) this should cause an issue but
for `v` in `[27, 28]` (everything else), the value of v will be reversed
(the resulting value of `v` will be `(27 & 1) + 27 == 28`, or `(28 & 1)
 + 27 == 27`).

tomashaber/raiden,hackaugusto/raiden,tomashaber/raiden,hackaugusto/raiden,tomashaber/raiden,tomashaber/raiden,tomashaber/raiden