fix(toxencryptsave): Wipe salt and passkey after usage.

[Green-Sky]

fixes #2925

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.06%. Comparing base (b21817d) to head (7cefa93).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
toxencryptsave/toxencryptsave.c	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2926      +/-   ##
==========================================
- Coverage   72.07%   72.06%   -0.01%     
==========================================
  Files         154      159       +5     
  Lines       31276    31305      +29     
==========================================
+ Hits        22542    22561      +19     
- Misses       8734     8744      +10     

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