Fix reading zip files with max length file comment.

If a zip file has a comment that is 65,535 characters long - which is a
valid length and the maximum allowable length - the initial read of the
archive fails to find the End of Central Directory Record and therefore
cannot read the rest of the file.

This commit fixes this by making sure that we look far enough back into
the file from the end to find the EoCDR. Test added to catch
regressions.

Fixes rubyzip#508.

lib/zip/central_directory.rb

@@ -7,9 +7,9 @@ class CentralDirectory
     END_OF_CDS             = 0x06054b50
     ZIP64_END_OF_CDS       = 0x06064b50
     ZIP64_EOCD_LOCATOR     = 0x07064b50
-    MAX_END_OF_CDS_SIZE    = 65_536 + 18
     STATIC_EOCD_SIZE       = 22
     ZIP64_STATIC_EOCD_SIZE = 56
+    MAX_END_OF_CDS_SIZE    = 65_535 + STATIC_EOCD_SIZE
 
     attr_reader :comment
 

test/file_test.rb

@@ -181,6 +181,11 @@ def test_open_buffer_without_block
     assert zf.entries.map(&:name).include?('zippedruby1.rb')
   end
 
+  def test_open_file_with_max_length_comment
+    # Should not raise any errors.
+    Zip::File.open('test/data/max_length_file_comment.zip')
+  end
+
   def test_cleans_up_tempfiles_after_close
     zf = ::Zip::File.new(EMPTY_FILENAME, create: true)
     zf.get_output_stream('myFile') do |os|