relax an assertion in packing external call args

[charles-cooper]

the number of args passed could be somewhere in between
base args + 0 kwargs and
base args + all kwargs

What I did

Fix #2520 and #2374

How I did it

#2447 partially fixed the bug but an assertion introduced in that PR was too aggressive.

How to verify it

vyper -f ir of the code in the issues

Description for the changelog

Fix calls to external functions involving kwargs

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2526 (3b09fab) into master (f31bd4a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2526   +/-   ##
=======================================
  Coverage   84.67%   84.67%           
=======================================
  Files          93       93           
  Lines        9307     9307           
  Branches     2099     2099           
=======================================
  Hits         7881     7881           
  Misses        940      940           
  Partials      486      486           

Impacted Files	Coverage Δ
vyper/old_codegen/external_call.py	94.33% <100.00%> (ø)

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