Fix uninitialized pointer in PBKDF2 macro

[kamilwaz]

This PR fixes a compilation error caused by an uninitialized pointer in the PBKDF2 macro.

Error:

===> /Users/kamilwaz/Workspace/esl/fast_pbkdf2.git/x-main/c_src/fast_pbkdf2.c:381:1: error: default initialization of an object of type 'HMAC_sha1_ctx_round *const restrict' leaves the object uninitialized [-Werror,-Wdefault-const-init-var-unsafe]
  381 | DECL_PBKDF2(sha1, SHA_CBLOCK, SHA_DIGEST_LENGTH, 3350 / SLICE)
      | ^
/Users/kamilwaz/Workspace/esl/fast_pbkdf2.git/x-main/c_src/fast_pbkdf2.c:213:47: note: expanded from macro 'DECL_PBKDF2'
  213 |         HMAC_CTX_ROUND(_name) *const restrict round_st;

Environment:

$ cc --version
Apple clang version 21.0.0 (clang-2100.0.123.102)
Target: arm64-apple-darwin25.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.31%. Comparing base (7302a0b) to head (3c718e7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #21   +/-   ##
=======================================
  Coverage   63.31%   63.31%           
=======================================
  Files           2        2           
  Lines         199      199           
  Branches       44       44           
=======================================
  Hits          126      126           
  Misses         73       73           

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[NelsonVides]

Good, this wasn't a const pointer really, it's modified just below 👍🏽