Prevent TypeError in read_central_directory_entries and use Zip::Error

[zelivans]

Bug found by fuzzing rubyzip.

In the #read_central_directory_entries method, @cdir_offset can be nil resulting in the call to io.seek raising an unhandled TypeError exception. It appears @cdir_offset is set in #read_e_o_c_d or #read_64_e_o_c_d, but I am not sure that it is considered a failure for it to be set to nil there, so in my fix I threw a Zip::Error exception just before trying to use the non-existent cdir_offset.

Raw crash

/home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/central_directory.rb:121:in `seek': no implicit conversion from nil to integer (TypeError)
	from /home/ariel/afl-kisaten/private/sandbox/rubyzip/rubyzip/lib/zip/central_directory.rb:121: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 (+4.5%) to 99.84% when pulling 0b1b650 on zelivans:fix_read_2 into d07b13a on rubyzip:master.

[hainesr]

Thanks for the PR.
I'm not sure that @cdir_offest can be nil, can it? It might end up with a wildly wrong value - which we handle a bit better in the current code than before - but once the two EOCD records are read it must be something other than nil I think. (The code has been updated quite a but since this PR.)