gtypemodule: Add assertions in finalize()
I've had leaks of this type on the Xfce panel for a long time, and I've finally taken the trouble to fix them:
==99586== 112 (16 direct, 96 indirect) bytes in 1 blocks are definitely lost in loss record 10,036 of 11,833
==99586== at 0x4841828: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==99586== by 0x57C4762: g_malloc (gmem.c:130)
==99586== by 0x57DDEEA: g_slist_prepend (gslist.c:282)
==99586== by 0x58EB873: g_type_module_register_type (gtypemodule.c:438)
==99586== by 0xDCF17F4: applications_menu_plugin_register_type (applicationsmenu.c:116)
==99586== by 0xDCF1813: xfce_panel_module_init (applicationsmenu.c:116)