Prevent NoMethodError crash when missing data in extra field

[zelivans]

Bug found by fuzzing rubyzip.

When creating an ExtraField from a binary string, in the #merge method there is no check for the binstr length, resulting in either id or len variables possibly being set to nil. I created a quick fix that raises a Zip::Error if either of these variables are nil to prevent an unhandled NoMethodError when using them.

Raw crash

/home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/extra_field.rb:14:in `extra_field_type_exist': undefined method `+' for nil:NilClass (NoMethodError)
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/extra_field.rb:44:in `merge'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/extra_field.rb:6:in `initialize'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:373:in `new'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:373:in `read_c_dir_extra_field'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:387:in `read_c_dir_entry'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:205:in `read_c_dir_entry'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/central_directory.rb:127:in `block in read_central_directory_entries'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/central_directory.rb:126:in `times'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/central_directory.rb:126:in `read_central_directory_entries'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/central_directory.rb:138:in `read_from_stream'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/file.rb:76:in `block in initialize'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/file.rb:75:in `open'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/file.rb:75:in `initialize'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/file.rb:97:in `new'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/file.rb:97:in `open'
	from zip.rb:3:in `<main>'

Reproduce with this file (extra_field.rb:14) or this one (extra_field.rb:12)

[coveralls]

Coverage increased (+0.005%) to 95.377% when pulling ede3baa on zelivans:fix_extra_1 into d07b13a on rubyzip:master.