Fix possible crash in TAR header parsing code.

Check that the iterator is valid before using its value, otherwise the
validity check was useless.

Closes #15888.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75642 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775