Check lastPreimage != nil before accessing the cached preimage

[mininny]

Description

The Fast VM implementation assumes the last preimage read has been cached, even if this is the first read. Moreover, the preimage key is always initialized to zeroes ([32]byte{}) while the cached preimage value is initialized to a null pointer.

That means if either there's an attempt to read the preimage value before writing a key, or if the key written is zero, an attempt to access the null pointer will cause a panic.

Neither scenario should happen in a correct Kona implementation -- reads should always be preceded by writing the corresponding key, and a key with zero value isn't valid. However this is the oracle's concern and should be delegated to it.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.89%. Comparing base (8ff9318) to head (1203fc4).
Report is 25 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #114   +/-   ##
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  Coverage   61.89%   61.89%           
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  Files          27       27           
  Lines        4091     4091           
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  Hits         2532     2532           
  Misses       1427     1427           
  Partials      132      132           

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[BlocksOnAChain]

@refcell We can merge this one if you are happy with it.
Auditors gave us the green light to merge it, but wanted to your view on it, before we merge?

[BlocksOnAChain]

@mininny we can merge it once you are ready to do it.