Fix memory leaks, missing property assignment, null dereferences, and unhandled errors in libmbdb

[Copilot]

Several bugs across the library ranging from critical memory corruption to memory leaks and unhandled failure paths. No existing test infrastructure.

Critical

• Missing property assignment (backup_file_create_from_record): Loop allocated each prop but never wrote it into properties[i], leaving the array full of garbage pointers — silent corruption on any record with properties.

// Before: prop allocated, configured, then silently dropped
mbdb_record_property_t* prop = malloc(sizeof(mbdb_record_property_t));
// ... fill prop ...
// ← nothing stored, properties[i] is uninitialized

// After
file->mbdb_record->properties[i] = prop;

Memory Leaks

• mbdb_parse: Four early-return paths allocated mbdb (and sub-fields header, data, records) without freeing on error. Fixed by calling mbdb_free(mbdb) before each early return.
• backup_open: backup_path leaked when mbdb_open() fails (mbdb_path was freed but backup_path was not).
• backup_file_create_from_record: file leaked when the mbdb_record malloc fails.
• mbdb_record_build: data_buf leaked when the size consistency check fails.

Null Pointer Dereferences

• backup_file_update_hash: Execution fell through after a failed fopen, passing a NULL FILE* to fread. Fixed by returning early after the error.
• mbdb_record_parse: properties array allocation and each prop allocation were unchecked; a failed malloc immediately dereferenced NULL on the next line. Added null checks with proper cleanup via mbdb_record_free.

Unhandled Error Returns

• backup_add_file_from_path: file_read() return value was ignored; a read failure left data as garbage passed into backup_file_create_with_data. Fixed by propagating the error.

Minor

• Removed five unused old_size local variables in mbdb_record setter functions (dead assignments, compiler warnings).
• Normalized allocation error messages to consistent casing/punctuation ("Allocation error\n").

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