Truncate BALANCE opcode parameter for witness recording

[jsign]

This PR was found by running our replay code in the latest changes.

The run had a panic at this line, since 20-len(addr) was negative which is an illegal index access. For context, we changed this line some days ago in #366.

That PR fix was fine since it fixes cases where addr length is less than 20. If we don't do 20-len(addr), the alignment is wrong. But the situation here is the inverse, if addr is longer than 20 bytes then we have this panic. From where this came from, it was the BALANCE opcode.

The value of BALANCE is a 32-byte value that must be truncated for a correct interpretation of the address (since no address can be 32 bytes anyway). This isn't always a problem since .Bytes() (old code line) truncates the value, removing the left-zeroes. But if the value is, for whatever reason, longer than 20 bytes, it can cause this panic. As in, most of the time the contract BALANCE parameter will be a 20-byte address (or smaller) since it's actually what it's expected semantically, but for weird cases where a potential arbitrary >20 byte value is provided is when this bug is triggered. (This is why this bug hasn't happened before)

[gballet]

Interestingly, I had a similar change in my access witness refactor PR, so if we had waited a couple more weeks we would have fixed this issue without knowing that it existed 😆 LGTM, and good job finding this.