k7zip: prevent crash when archive has no modification times for files

https://crash-reports.kde.org/organizations/kde/issues/64602/?project=128&query=is%3Aunresolved+issue.priority%3A%5Bhigh%2C+medium%5D&referrer=issue-stream&stream_index=16

Thread 32628 Crashed:
0   libKF6Archive.so.6.5.0          0x72276776912f      K7Zip::openArchive (k7zip.cpp:2687)
1   libkioarchive6.so.6.0.0         0x72276f127bc6      ArchiveProtocolBase::checkNewFile (kio_archivebase.cpp:151)
2   libkioarchive6.so.6.0.0         0x72276f12ac80      ArchiveProtocolBase::stat (kio_archivebase.cpp:313)
3   libKF6KIOCore.so.6.5.0          0x72276f001394      KIO::WorkerSlaveBaseBridge::stat (workerbase_p.h:111)
4   libKF6KIOCore.so.6.5.0          0x72276effcc9d      KIO::SlaveBase::dispatch (slavebase.cpp:1208)
5   libKF6KIOCore.so.6.5.0          0x72276f0a7c1e      KIO::SlaveBase::dispatchLoop (slavebase.cpp:350)
6   archive.so                      0x72276f132d3f      kdemain (kio_archive.cpp:45)
7   kioworker                       0x60ae0f45e22c      main (kioworker.cpp:144)
8   libc.so.6                       0x72276e634e07      __libc_start_call_main (libc_start_call_main.h:58)
9   libc.so.6                       0x72276e634ecb      __libc_start_main (libc-start.c:360)
10  kioworker                       0x60ae0f45e4f4      _start