Bugfix in ConcatLayer
Bugfix in ConcatLayer

Fixes a bug in ConcatLayer that occurs in backward() when
state.blobs_diff[1] is a NullBlob. Also avoids creating unnecessary
Blobs in state.blobs_diff during setup().

dplarson/Mocha.jl,JuliaPackageMirrors/Mocha.jl,JuliaPackageMirrors/Mocha.jl,dplarson/Mocha.jl