fix fill when length = 0

[Ellipse0934]

As reported in #81 , CUDA.jl . Currently constructors such as CUDA.zeros(T, 0) would fail for the types not handled by CUDA memset (64-bit numbers not supported).

Previously

julia> T = Float64
julia> CuArrays.zeros(T, 0)
ERROR: AssertionError: total_threads > 0
Stacktrace:
 [1] #gpu_call#1 at /scratch/snx3000tds/omlins/9_soft_install_julia_1_4/software/Julia/1.4.1-CrayGNU-19.10-cuda-10.1/extensions/packages/GPUArrays/OXvxB/src/device/execution.jl:60 [inlined]
 [2] gpu_call at /scratch/snx3000tds/omlins/9_soft_install_julia_1_4/software/Julia/1.4.1-CrayGNU-19.10-cuda-10.1/extensions/packages/GPUArrays/OXvxB/src/device/execution.jl:46 [inlined]
 [3] fill!(::CuArray{Float64,1,Nothing}, ::Int64) at /scratch/snx3000tds/omlins/9_soft_install_julia_1_4/software/Julia/1.4.1-CrayGNU-19.10-cuda-10.1/extensions/packages/GPUArrays/OXvxB/src/host/construction.jl:12
 [4] zeros(::Type{T} where T, ::Int64, ::Vararg{Int64,N} where N) at /scratch/snx3000tds/omlins/9_soft_install_julia_1_4/software/Julia/1.4.1-CrayGNU-19.10-cuda-10.1/extensions/packages/CuArrays/l0gXB/src/array.jl:337
 [5] top-level scope at REPL[129]:1

Now:

julia> CUDA.zeros(Float64, 0)
0-element CuArray{Float64,1}

[maleadt]

Thanks!
bors r+

[bors]

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