g_malloc(0) and g_malloc0(0) return NULL; simplify

g_malloc(0) and g_malloc0(0) return NULL; simplify

Once upon a time, it was decided that qemu_malloc(0) should abort.
Switching to glib retired that bright idea.  Some code that was added
to cope with it (e.g. in commits 702ef63, b76b6e9) is still around.
Bury it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>