Cleanup use of mut ptr

[justsmth]

Description of changes:

• Ensuring we have a mut reference when providing a mut pointer.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.67%. Comparing base (c358484) to head (2d7b1b7).
Report is 18 commits behind head on main.

Files	Patch %	Lines
aws-lc-rs/src/cbb.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #430      +/-   ##
==========================================
- Coverage   95.80%   92.67%   -3.13%     
==========================================
  Files          61       61              
  Lines        8143     8174      +31     
  Branches        0     8174    +8174     
==========================================
- Hits         7801     7575     -226     
- Misses        342      357      +15     
- Partials        0      242     +242     

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