Fix vector-based CompositeMap mul kernel

[dkarrasch]

Fixes #171 (comment).

[codecov]

Codecov Report

Merging #175 (6348723) into master (f8eaf98) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #175   +/-   ##
=======================================
  Coverage   99.64%   99.65%           
=======================================
  Files          14       14           
  Lines        1130     1150   +20     
=======================================
+ Hits         1126     1146   +20     
  Misses          4        4           

Impacted Files	Coverage Δ
src/composition.jl	100.00% <100.00%> (ø)

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