New StructGuard variant to support non-void return types.

[pattivacek]

gcc9 explicitly complains about the previous solution of casting the function parameter.

[codecov-commenter]

Codecov Report

Merging #1714 into master will increase coverage by 3.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1714      +/-   ##
==========================================
+ Coverage   74.47%   77.56%   +3.09%     
==========================================
  Files         185      185              
  Lines       13699    13144     -555     
==========================================
- Hits        10202    10195       -7     
+ Misses       3497     2949     -548     

Impacted Files	Coverage Δ
src/libaktualizr/utilities/utils.h	93.33% <ø> (ø)
src/libaktualizr/utilities/utils.cc	87.80% <100.00%> (+19.26%)	⬆️
src/libaktualizr/storage/sqlstorage_base.h	60.00% <0.00%> (-40.00%)	⬇️
src/libaktualizr/storage/sqlstorage_base.cc	76.19% <0.00%> (-4.09%)	⬇️
src/libaktualizr/package_manager/ostreemanager.cc	79.77% <0.00%> (-0.74%)	⬇️
src/libaktualizr-posix/ipuptanesecondary.cc	81.31% <0.00%> (+0.98%)	⬆️
src/libaktualizr/storage/sqlstorage.cc	57.34% <0.00%> (+1.14%)	⬆️
src/libaktualizr-posix/asn1/asn1_message.h	64.51% <0.00%> (+5.69%)	⬆️
...c/virtual_secondary/partialverificationsecondary.h	30.76% <0.00%> (+5.76%)	⬆️
src/virtual_secondary/managedsecondary.h	100.00% <0.00%> (+7.69%)	⬆️
... and 14 more

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