fix off-by-one in dynamic array length calculation

[charles-cooper]

What I did

fix #2820

How I did it

How to verify it

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2821 (5f30d0c) into master (c270363) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2821      +/-   ##
==========================================
+ Coverage   87.47%   87.52%   +0.05%     
==========================================
  Files          94       94              
  Lines       10003    10007       +4     
  Branches     2480     2480              
==========================================
+ Hits         8750     8759       +9     
+ Misses        785      782       -3     
+ Partials      468      466       -2     

Impacted Files	Coverage Δ
vyper/semantics/types/indexable/sequence.py	88.18% <100.00%> (ø)
vyper/semantics/types/bases.py	84.53% <0.00%> (+0.51%)	⬆️
vyper/semantics/types/function.py	87.10% <0.00%> (+0.99%)	⬆️
vyper/ast/validation.py	77.77% <0.00%> (+4.44%)	⬆️

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[fubuloubu]

Needs tests