validate binary plist offset_size to prevent crashes

[wtn]

Fixes crashes when parsing malformed binary plist files with invalid offset_size values.

Changes

• Add validation that offset_size is in valid range [1,2,3,4]
• Add nil check for coded_offset_table before calling bytesize
• Add tests for invalid offset_size and offset table overflow

Background

The binary parser would crash with TypeError or NoMethodError when given malformed plist files with invalid offset_size values (e.g., 5) or when fd.read returned nil.