fix: calldatasize < 4 reverting instead of going to fallback

[charles-cooper]

in the case that a selector matches calldata with less than 4 bytes, it currently will revert instead of going to the fallback. this commit fixes the behavior in the special case where no data besides the method id is expected in the calldata.

note this regression was originally introduced in 02339df.

What I did

How I did it

in the case where expected calldatasize == 4, we do not expect any extra data. therefore, if calldatasize < 4, we can assume the selector is wrong and go to fallback instead of reverting as if we were doing calldata validation.

How to verify it

see the test. calling fow() with the leading 3 bytes of the selector (i.e., 0xd88e0b instead of 0xd88e0b00) currently reverts on master, while on this branch it correctly goes to the fallback function.

Commit message

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Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #3408 (45b9fce) into master (97ff017) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3408      +/-   ##
==========================================
- Coverage   88.96%   88.90%   -0.07%     
==========================================
  Files          86       86              
  Lines       10765    10804      +39     
  Branches     2445     2454       +9     
==========================================
+ Hits         9577     9605      +28     
- Misses        786      801      +15     
+ Partials      402      398       -4     

Impacted Files	Coverage Δ
.../codegen/function_definitions/external_function.py	100.00% <100.00%> (ø)

... and 15 files with indirect coverage changes

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