Fix submitted that evaluates the ECD structure and validates the first CD entry. The fix also handles empty zipfiles.

IMO the purpose of this API is to *quickly* verify that the file is a valid zipfile. With this fix, the API only reads another 46 bytes of data (after a seek, of course). This should still qualify as "quick", especially after having potentially read 64k of data.

Perhaps a full zip validator would be appropriate in addition to is_zipfile. That would be more appropriate as a full feature rather than in this bugfix.