dnsdist: Fix a few warnings from 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 10845334446

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 95 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+3.4%) to 64.677%

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Files with Coverage Reduction	New Missed Lines	%
pdns/backends/gsql/gsqlbackend.hh	2	97.71%
pdns/rcpgenerator.cc	2	89.68%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/packethandler.cc	4	72.43%
pdns/misc.cc	4	63.37%
pdns/recursordist/test-syncres_cc1.cc	8	89.38%
pdns/recursordist/taskqueue.cc	11	34.15%
ext/json11/json11.cpp	12	62.72%
pdns/recursordist/syncres.cc	15	79.37%
pdns/recursordist/rec-taskqueue.cc	34	40.14%

Totals
Change from base Build 10843570514:	3.4%
Covered Lines:	124814
Relevant Lines:	162271

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💛 - Coveralls