Fix several minor safety issues

[oschwald]

• Reject unsupported sockaddr families
• Reject empty metadata sections
• Reject search tree records in separator
• Return an error for invalid search nodes

[gemini-code-assist]

Code Review

This pull request addresses several security and stability issues, including an out-of-bounds read in MMDB_lookup_sockaddr, improved search-tree validation to reject records pointing into the separator, and fixes for metadata parsing. It also introduces new tests for these scenarios. The review feedback points out that the newly added test files are not cross-platform and will likely fail to build on Windows due to reliance on POSIX-specific headers and functions.