Fixing trace in volBoundary2obj

[dcoeurjo]

PR Description

Fixing segfault in volBoiundary2obj

Checklist

• Doxygen documentation of the code completed (classes, methods, types, members...).
• Main tool doxygen documentation (following existing documentation of DGtalTools documentation.
• Check if it follows the tools structure described in CONTRIBUTING.md
• New entry in the ChangeLog.md added.
• Update the readme with potentially a screenshot of the tools if it applies.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).

[kerautret]

fine,thanks, strange that the segfault was not given before. mering