Fix indexing for InfStepRange

[jishnub]

This works around a 0 * Inf multiplication while indexing an InfStepRange.
Currently, on master,

julia> v = InfiniteArrays.InfStepRange(InfiniteCardinal{0}(), InfiniteCardinal{0}())
ℵ₀:ℵ₀:+∞

julia> v[1]
ERROR: ArgumentError: 0 is non-positive
Stacktrace:
 [1] *
   @ ~/.julia/packages/Infinities/Y1fpA/src/cardinality.jl:121 [inlined]
 [2] getindex(v::InfiniteArrays.InfStepRange{InfiniteCardinal{0}, InfiniteCardinal{0}}, i::Int64)
   @ InfiniteArrays ~/Dropbox/JuliaPackages/InfiniteArrays.jl/src/infrange.jl:202
 [3] top-level scope
   @ REPL[33]:1

After this PR

julia> v[1]
ℵ₀

[codecov]

Codecov Report

Merging #102 (f067f6a) into master (5a4acbf) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   85.01%   85.03%   +0.02%     
==========================================
  Files           4        4              
  Lines         654      655       +1     
==========================================
+ Hits          556      557       +1     
  Misses         98       98              

Impacted Files	Coverage Δ
src/infrange.jl	87.82% <100.00%> (+0.04%)	⬆️

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