Glib::OptionGroup: Fix memory leaks
Glib::OptionGroup: Fix memory leaks

* glib/src/optiongroup.ccg: CppOptionEntry::set_c_arg_default(),
convert_c_to_cpp(), release_c_arg(): Fix memory leaks for string-valued
options. Bug #745173.

GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm