Add more consistency to rnp_op_verify_st/rnp_op_generate_st constructors.

[ni4]

As per this review comment: #2309 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.17%. Comparing base (dc13a59) to head (ee1bce5).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib/crypto/ecdsa.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2310   +/-   ##
=======================================
  Coverage   85.17%   85.17%           
=======================================
  Files         118      118           
  Lines       22793    22795    +2     
=======================================
+ Hits        19414    19416    +2     
  Misses       3379     3379           

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