Fix allocation issue in ManifoldDiff.jl

[mateuszbaran]

This fixes the issue discovered here: https://github.com/JuliaManifolds/ManifoldDiff.jl/pull/18 .

[codecov[bot]]

Codecov Report

Merging #152 (6cc0c43) into master (f3f906d) will decrease coverage by 0.12%. The diff coverage is 100.00%.

:exclamation: Current head 6cc0c43 differs from pull request most recent head 3da980a. Consider uploading reports for the commit 3da980a to get more accurate results

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage   99.96%   99.84%   -0.12%     
==========================================
  Files          19       19              
  Lines        2529     2529              
==========================================
- Hits         2528     2525       -3     
- Misses          1        4       +3     

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

... and 1 file with indirect coverage changes

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