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 702ef63f, b76b6e95) 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>