Remove use of nullptr in test.
Add a macro _GLIBCXX_DEBUG on debug mode to enable libstdc++ debug mode.
Merge pull request #889 from gwtnb/libstdc++-debug

Add a macro _GLIBCXX_DEBUG on debug mode to enable libstdc++ debug mode.
Merge pull request #888 from gwtnb/remove-nullptr-in-test

Remove use of nullptr in test.