[PJK]

Summary

• Fix integer overflow when summing chunk lengths in cbor_copy_definite() for indefinite bytestrings and strings
• Crafted chunks with lengths summing past SIZE_MAX would silently wrap around, causing a small _cbor_malloc followed by heap buffer overflow in memcpy
• Use existing _cbor_safe_to_add() to detect overflow and return NULL

Test plan

• Added test_definite_indef_bytestring_chunk_length_overflow — verifies cbor_copy_definite() returns NULL when bytestring chunk lengths would overflow
• Added test_definite_indef_string_chunk_length_overflow — same for strings
• All 26 existing test binaries pass
• clang-format clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d3e40ca) to head (27399d4).
⚠️ Report is 36 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #387   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         1653      1657    +4     
=========================================
+ Hits          1653      1657    +4     

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