TensorMath.lua: zero init result tensor
TensorMath.lua: zero init result tensor

A function like `torch.mm` stores its output into a result tensor (res)
and uses the `addmm` function behind the scenes. `addmm` performs multi-
plications like res = beta x res + alpha x sum with beta=0 and alpha=1
here. If res is not initialized NaN values could occur resulting on NaN
output values.

The same applies for `torch.bmm` and `torch.mv`.
Moodstocks/torch7,yozw/torch7,clementfarabet/torch7,adamlerer/torch7,wickedfoo/torch7,nicholas-leonard/torch7,clementfarabet/torch7,colesbury/torch7,nicholas-leonard/torch7,szagoruyko/torch7,wickedfoo/torch7,adamlerer/torch7,Atcold/torch7,diz-vara/torch7,colesbury/torch7,wgapl/torch7,Moodstocks/torch7,Atcold/torch7,yozw/torch7,diz-vara/torch7,szagoruyko/torch7,wgapl/torch7