Fix a mis-firing CentralDirectory test.

`test_read_from_truncated_zip_file` was not testing what it thought it
was. It was testing whether we caught an out-of-bounds cdir offset, not
whether we caught a corrupted cdir entry.

This commit embraces the actual behaviour and tests that we catch an
out-of-bounds error for both standard `IO`s and `StringIO`s.

lib/zip/central_directory.rb

@@ -149,11 +149,16 @@ module Zip
     end
 
     def read_central_directory_entries(io) #:nodoc:
+      # `StringIO` doesn't raise `EINVAL` if you seek beyond the current end,
+      # so we need to catch that *and* query `io#eof?` here.
+      eof = false
       begin
         io.seek(@cdir_offset, IO::SEEK_SET)
       rescue Errno::EINVAL
-        raise Error, 'Zip consistency problem while reading central directory entry'
+        eof = true
       end
+      raise Error, 'Zip consistency problem while reading central directory entry' if eof || io.eof?
+
       @entry_set = EntrySet.new
       @size.times do
         entry = Entry.read_c_dir_entry(io)

test/central_directory_test.rb

@@ -26,15 +26,22 @@ class ZipCentralDirectoryTest < MiniTest::Test
   rescue ::Zip::Error
   end
 
-  def test_read_from_truncated_zip_file
-    fragment = ''
-    File.open('test/data/testDirectory.bin', 'rb') { |f| fragment = f.read }
-    fragment.slice!(12) # removed part of first cdir entry. eocd structure still complete
-    fragment.extend(IOizeString)
-    entry = ::Zip::CentralDirectory.new
-    entry.read_from_stream(fragment)
-    raise 'ZipError expected'
-  rescue ::Zip::Error
+  def test_read_eocd_with_wrong_cdir_offset_from_file
+    ::File.open('test/data/testDirectory.bin', 'rb') do |f|
+      assert_raises(::Zip::Error) do
+        cdir = ::Zip::CentralDirectory.new
+        cdir.read_from_stream(f)
+      end
+    end
+  end
+
+  def test_read_eocd_with_wrong_cdir_offset_from_buffer
+    ::File.open('test/data/testDirectory.bin', 'rb') do |f|
+      assert_raises(::Zip::Error) do
+        cdir = ::Zip::CentralDirectory.new
+        cdir.read_from_stream(StringIO.new(f.read))
+      end
+    end
   end
 
   def test_write_to_stream