Cleanup static analyzer issues

[kcgen]

This PR fixes issues flagged by the Clang static analyzer, Coverity, and PVS Studio. Suggest reviewing commit-by-commit, for which the following provides more background.

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Uninitialized member variables, flagged by GCC and PVS Studio

../submodules/loguru/loguru.cpp:1491:9: warning: 'loguru::LogScopeRAII::_indent_stderr' should be initialized in the member initialization list
../submodules/loguru/loguru.cpp:1491:9: warning: 'loguru::LogScopeRAII::_start_time_ns' should be initialized in the member initialization list
../submodules/loguru/loguru.cpp:1630:16: warning: 'loguru::StringStream::str' should be initialized in the member initialization list
../submodules/loguru/loguru.cpp:1725:9: warning: 'loguru::EcEntryBase::_previous' should be initialized in the member initialization list 

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Unused position increment (clarify intent), flagged by Clang's static analyzer

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Simplify empty std::string operations (1/2), flagged by PVS Studio

Simplify empty std::string operations (2/2), flagged by PVS Studio

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Potential nullptr dereference, flagged by PVS Studio

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Clarify criteria by removing redundancy, flagged by PVS Studio

If it makes sense to keep keep them, I could see them be valid as asserts orCHECK_F`s -- let me know.

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Virtualize EcEntryBase's destructor so it's called by derived classes, flagged by LGTM

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Explicitly ignore sigaction's return value, flagged by Coverity

[kcgen]

Re-sync'd with latest changes in master.

[kcgen]

Recently merged PR#180 introduce two new PVS Studio warnings:

This branch now fixes these as well, in commit: https://github.com/dosbox-staging/loguru/commit/0155fe9552c470754e1c1919d3cf7f7df90a21ff