gthread: Fix "zero as null pointer" warning
gthread: Fix "zero as null pointer" warning

When compiling a program using glib with -Wzero-as-null-pointer-constant
warnings enabled, the compiler warns about this type check in the
g_once_init_enter macro. Fix by replacing "0" with "NULL".

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib