Fix SpatialClassNLLCriterion when using OMP
Fix SpatialClassNLLCriterion when using OMP

Although the `b` variable is automatically private, the `elem`
loop variable is not private.

Test: change test.lua to run with more than one thread

eriche2016/nn,joeyhng/nn,diz-vara/nn,colesbury/nn,nicholas-leonard/nn,sagarwaghmare69/nn,kmul00/nn,apaszke/nn