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
Signed-off-by: Marian 'VooDooMan' Meravy <vdm113@gmail.com>

Conflicts:
	src/common/tarstrm.cpp
patched for ICC

Signed-off-by: Marian 'VooDooMan' Meravy <vdm113@gmail.com>