Prevent TypeError when central directory offset is nil, raise Zip::Er…

…ror instead

lib/zip/central_directory.rb

@@ -117,6 +117,7 @@ def read_e_o_c_d(buf) #:nodoc:
     end
 
     def read_central_directory_entries(io) #:nodoc:
+      raise Error, 'Zip consistency problem with central directory entry offset' if @cdir_offset.nil?
       begin
         io.seek(@cdir_offset, IO::SEEK_SET)
       rescue Errno::EINVAL