Update SharpZipWriter.cs

- entry is null
_zipFile hasnt been updated with _zipFile.CommitUpdate() yet. Exception "Cannot find central directory" is then thrown by trying to set entry.Comment

- MemoryStream sourceFile.Position is at the end
nothing will be written into the zipfile, because sourceFile.Read(buffer, 0, readSize) returns 0 Bytes. Exception "Failed to copy bytes expected xx read 0" is thrown by _zipFile.CommitUpdate() at ICSharpCode.SharpZipLib.Zip.ZipFile.CopyBytes