fix: ecrecover() edge case

[charles-cooper]

What I did

fix an edge case that was not coverd by 019a37a

How I did it

How to verify it

Commit message

this commit fixes an edge case in `ecrecover()` that was not covered by
019a37ab98ff5. in the case that one of the arguments to ecrecover writes
to memory location 0, and the signature is invalid, `ecrecover()` could
return the data written by the argument. this commit fixes the issue by
allocating fresh memory for the output buffer (which won't be written to
by evaluating any of the arguments unless the memory allocator is
broken).

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #3586 (3a687b9) into master (2c21eab) will decrease coverage by 0.08%.
Report is 4 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3586      +/-   ##
==========================================
- Coverage   89.05%   88.98%   -0.08%     
==========================================
  Files          85       85              
  Lines       11378    11390      +12     
  Branches     2586     2586              
==========================================
+ Hits        10133    10135       +2     
- Misses        821      830       +9     
- Partials      424      425       +1     

Files Changed	Coverage Δ
vyper/builtins/functions.py	90.55% <100.00%> (+0.08%)	⬆️

... and 3 files with indirect coverage changes

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