std.zip: zip64 should take size values from extrafields into account

[dlangBugzillaToGithub]

bugzilla (@berni44) reported this on 2019-10-10T18:07:28Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=20289

Description

If the file is in zip64 format, it is possible, that some size/offset informations are not stored in the record but in some extra data fields. In the original place are then 0xff bytes. The current implementation does not consider this and takes the 0xff bytes as size value.