DGraph: Fix dll boundary prob using qPrintable() instead of toStdString()
DGraph: Fix dll boundary prob using qPrintable() instead of toStdString()

The following page provides more details regarding dll boundary problem
which could appear when for example a debug executable is linked
against a release library and when memory allocated in one is freed in
the other.

See http://stackoverflow.com/questions/1227653/linking-against-library-in-release-and-exe-in-debug-crashes-in-visual-studio

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