The FBX exporter was unconditionally dereferencing a map iterator (`tp_elem`)
even when the key was not found (i.e., `tp_elem == tpath_by_image.end()`).
This resulted in a stack-use-after-scope error when accessing `tp_elem->second`
to populate "FileName" and "RelativeFilename" nodes, as dereferencing the
end iterator of the map accessed invalid stack memory (the map's sentinel).
The code already correctly initialized a local `tfile_path` variable based
on whether the iterator was valid. This patch updates the `AddChild` calls
to use `tfile_path` instead of dereferencing the potentially invalid iterator.
Fixes: https://issues.oss-fuzz.com/issues/465494996
Co-authored-by: CodeMender <codemender-patching@google.com>
Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>
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