It could've never been uninitialized in this code but the code flow is
not obvious to the compiler. Initialize it to NULL and for clarity also
add an assertion that it is not NULL anymore on usage.

In file included from ../glib/glib.h:62,
                 from ../gobject/gbinding.h:28,
                 from ../glib/glib-object.h:23,
                 from ../gio/gioenums.h:28,
                 from ../gio/giotypes.h:28,
                 from ../gio/giomodule.h:28,
                 from ../gio/giomodule.c:25:
../gio/giomodule.c: In function ‘_g_io_module_get_default’:
../glib/gmessages.h:343:25: warning: ‘extension’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 #define g_debug(...)    g_log (G_LOG_DOMAIN,         \
                         ^~~~~
../gio/giomodule.c:912:17: note: ‘extension’ was declared here
   GIOExtension *extension, *preferred;
                 ^~~~~~~~~