Once upon a time, it was decided that qemu_realloc(ptr, 0) should
abort.  Switching to glib retired that bright idea.  A bit of code
that was added to cope with it (commit 3e372cf8) is still around.  Bury
it.

See also commit 6528499f.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>