Fix panic on folders with zero data blocks

[ozzieba]

Summary

Fixes #37 -- FolderReader::seek_to_uncompressed_offset panics with "index out of bounds" when a malformed CAB file has a folder with num_data_blocks == 0 but file entries that reference it with a non-zero uncompressed offset.

Changes:

• Add a bounds check before accessing self.data_blocks[self.current_block_index]: if data_blocks is empty, return io::Error(InvalidData) instead of panicking
• Add a bounds check after incrementing self.current_block_index in the seek loop: if the index exceeds num_data_blocks, return io::Error(InvalidData) instead of potentially panicking on subsequent iterations
• Add a test with a crafted malformed CAB binary (zero data blocks, non-zero file offset) that verifies the error is returned correctly

Files changed:

• src/folder.rs -- two bounds checks in seek_to_uncompressed_offset
• src/cabinet.rs -- one new test

All existing tests continue to pass.

[mdsteele]

Thanks, and sorry for the slow reply. The linter is complaining, but looks good otherwise.