[bugfix] Fix crash when using syslog logging handler with the -v option

[vkarak]

Fixes #763.

[vkarak]

@jenkins-cscs retry none

[codecov-io]

Codecov Report

Merging #808 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #808      +/-   ##
==========================================
- Coverage   91.57%   91.55%   -0.03%     
==========================================
  Files          80       80              
  Lines        9975     9975              
==========================================
- Hits         9135     9133       -2     
- Misses        840      842       +2

Impacted Files	Coverage Δ
reframe/core/logging.py	83.54% <ø> (ø)	⬆️
reframe/core/config.py	84.07% <0%> (-1.77%)	⬇️

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