Fix logger singleton

[RainerKuemmerle]

Fix #732

[codecov]

Codecov Report

Merging #736 (3374b0c) into master (f559e4b) will decrease coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #736      +/-   ##
==========================================
- Coverage   50.96%   50.96%   -0.01%     
==========================================
  Files         271      271              
  Lines       10985    10989       +4     
==========================================
+ Hits         5599     5601       +2     
- Misses       5386     5388       +2     

Files	Coverage Δ
g2o/stuff/logger.h	100.00% <ø> (ø)
g2o/stuff/logger.cpp	29.16% <66.66%> (+4.16%)	⬆️

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