Prevent NoMethodError when @name is nil in entry

[zelivans]

Bug found by fuzzing rubyzip.

In the #name_is_directory? method the @name variable is used to check if the entry is a directory. This results in an unhandled NoMethodError when @name is nil. In this case @name was set in #read_c_dir_entry, where I added a quick fix by setting it to an empty string when it would be set to nil.

I do think some more serious refactoring might be needed to prevent bugs like this one from happening with rubyzip but for now this solves this precise crash.

Raw crash

/home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:109:in `name_is_directory?': undefined method `end_with?' for nil:NilClass (NoMethodError)
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:346:in `set_ftype_from_c_dir_entry'
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/entry.rb:390: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

[coveralls]

Coverage increased (+4.6%) to 100.0% when pulling a2cba27 on zelivans:fix_read_3 into d07b13a on rubyzip:master.