fix: thread safety selecting codec

[mayeut]

libbase64 was re initialized on each call. It was likely harmless, at least on most common platform. It's also a bit more performant although not significant.

[codspeed-hq]

Merging this PR will not alter performance

✅ 48 untouched benchmarks

---

_{Comparing thread-safety (f2dee31) with master (49e273f)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (49e273f) to head (f2dee31).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1041   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files          11       11           
  Lines        1927     1928    +1     
  Branches      307      308    +1     
=======================================
+ Hits         1919     1920    +1     
  Partials        8        8           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.