compile_lll.py: inconsistency in checking invalid cases

[daejunpark]

What's your issue about?

Inconsistency in checking invalid cases in compile_lll.py.

For the with variable lookup, it rejects immediately when the with statement is too deep:
https://github.com/ethereum/viper/blob/b1fe12ddd8706ef3340a103916e3d3f2cc95ceef/viper/compile_lll.py#L50-L54
but for the with variable update, it does not check at all, which may lead to generate an invalid/wrong opcode (e.g., if the depth is 17, then internally it will generate SWAP17 (doesn't exist) that will end up being encoded as 0xa0, which is LOG0):
https://github.com/ethereum/viper/blob/b1fe12ddd8706ef3340a103916e3d3f2cc95ceef/viper/compile_lll.py#L55-L60

However, I couldn't find an example program that leads to such a deep with statement. It might not be possible at this moment, but it would be good to fix for later.

How can it be fixed?

Add the following check in the case of code.value == "set":

     if height - withargs[code.args[0].value] > 16: 
         raise Exception("With statement too deep") 

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[DavidKnott]

@daejunpark Thanks for brining this up, I'll make a pr to fix this.