dnsdist: Fix possible low impact performance issues reported by Coverity

[rgacogne]

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 12888905352

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 3 of 8 (37.5%) changed or added relevant lines in 3 files are covered.
• 50 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.02%) to 64.719%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/dnsdist-lua.cc	0	1	0.0%
pdns/dnsdistdist/dnsdist-configuration-yaml.cc	2	6	33.33%

Files with Coverage Reduction	New Missed Lines	%
pdns/packethandler.cc	1	72.44%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
pdns/recursordist/test-syncres_cc2.cc	3	89.03%
pdns/dnsdistdist/dnsdist-carbon.cc	9	62.13%
pdns/recursordist/rec-tcp.cc	10	66.07%
ext/json11/json11.cpp	12	62.72%
modules/lmdbbackend/lmdbbackend.cc	14	72.75%

Totals
Change from base Build 12884379895:	0.02%
Covered Lines:	127804
Relevant Lines:	166432

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💛 - Coveralls