Add missing initialization for g_auto* variables

When building Fedora package, I see many "may be used uninitialzed" warnings. I am not sure why but I don't see those warnings locally even when -Wmaybe-uninitialized is used. Anyway, this is because the g_autofree and g_autoptr variables are not explicitely initialized as mentioned in the documentation. Let's initialize them to get rid of those warnings.