Avoid zero division in decompose

[blegat]

This is making CDDLib and Polyhedra tests fail in Julia v1.6 due to the change fixed in JuliaLang/julia#40551

[codecov]

Codecov Report

Merging #248 (d851b8a) into master (57858dc) will increase coverage by 1.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   76.46%   78.12%   +1.66%     
==========================================
  Files          39       39              
  Lines        2961     3204     +243     
==========================================
+ Hits         2264     2503     +239     
- Misses        697      701       +4     

Impacted Files	Coverage Δ
src/vecrep.jl	100.00% <ø> (ø)
src/decompose.jl	73.38% <100.00%> (+2.59%)	⬆️
src/indices.jl	96.92% <0.00%> (-3.08%)	⬇️
src/comp.jl	91.30% <0.00%> (-1.01%)	⬇️
src/planar.jl	100.00% <0.00%> (ø)
src/mixedrep.jl	100.00% <0.00%> (ø)
src/polyhedron.jl	100.00% <0.00%> (ø)
src/triangulation.jl	100.00% <0.00%> (ø)
src/interval.jl	99.13% <0.00%> (+0.02%)	⬆️
... and 26 more

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