Fix build failure in linux

Fixes marmelroy/Zip#277 It seems that Swift is unwilling to implicitly unwrap the optional `filePointer: UnsafeMutablePointer?` when passing it to fwrite in linux, you can see this build failure in building a dependand library [here](https://github.com/csknns/wallet/actions/runs/14143909393/job/39628890252). So I unwrap the filePointer before writing bytes. Also I improved the while loop that reads bytes from the zip file until there are no more, the condition before was `while filePointer != nil` that was always true, not the condition is that read method returns that zero bytes where read.