Fix memory leaks when returning strings from C++ to Rust

[scouten-adobe]

Changes in this pull request

Fixes #102.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov]

Codecov Report

Merging #104 (63c818c) into main (c697230) will increase coverage by 0.15%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   95.73%   95.89%   +0.15%     
==========================================
  Files           7        7              
  Lines         704      730      +26     
==========================================
+ Hits          674      700      +26     
  Misses         30       30              

Impacted Files	Coverage Δ
src/xmp_date_time.rs	97.50% <66.66%> (-0.10%)	⬇️
src/ffi.rs	100.00% <100.00%> (ø)
src/xmp_meta.rs	100.00% <100.00%> (ø)

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