lguest: fix occasional crash in example launcher.

commit ca16f580a5db7e60bfafe59a50bb133bd3347491 upstream.

We usually got away with ->next on the final entry being NULL, but it
finally bit me.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>