Indexing off-by-one?

[Max Guise]

Am I missing a case in which the pa+1 is needed instead of pa for the loop counter?  Especially with c->used set to pa, looks like the intent may have been for this counter to be set to pa as well.

https://github.com/libtom/libtommath/blob/master/bn_fast_s_mp_mul_digs.c#L90