Have mvn.rnd support *, 1xD, * calls as if they were *, D, *

As I was adding tests where mu is 1xD (instead of NxD or D), torch
has suddenly segfaulted on me -- both qth and th.
@d11, can you please see if you have the same problem with your install? 
I'm at a loss to understand what's going on.

(edit: fixed, see below)