Add check for regular file

[RainerKuemmerle]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 31.80%. Comparing base (e16cfc2) to head (8aae267).
Report is 5 commits behind head on master.

Files	Patch %	Lines
g2o/apps/g2o_viewer/run_g2o_viewer.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #786       +/-   ##
===========================================
- Coverage   50.89%   31.80%   -19.10%     
===========================================
  Files         271      427      +156     
  Lines       10967    21083    +10116     
  Branches        0     2316     +2316     
===========================================
+ Hits         5582     6706     +1124     
- Misses       5385    14024     +8639     
- Partials        0      353      +353     

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