gio/ginetaddress.c: Fix Windows XP inet_pton() Emulation

We need to be more careful when we try to assign values to gpointers, so
that means we have to assign the value to the properly-dereference
gpointer, so that the assigned value will be retained after the function
returns.  This code will be dropped soon, but it is done for XP
compatibility's sake for 2.44.

Should fix the issue reported in bug 730352 comment #24.