dnsdist: Fix performance inefficiencies reported by Coverity

[rgacogne]

Short description

Fixes Coverity 490091, 490092, 490093 and 490094.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI policy, and added a "Signed-off-by" to my commits
• 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 17757926936

Details

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• 59 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.1%) to 66.034%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/dnsdist-lua-ffi.cc	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
pdns/iputils.hh	1	78.97%
pdns/validate.cc	1	68.5%
pdns/misc.cc	2	61.76%
pdns/dnsdistdist/dnsdist-tcp.cc	4	77.34%
pdns/recursordist/syncres.cc	4	81.14%
pdns/recursordist/pdns_recursor.cc	9	73.89%
pdns/recursordist/lwres.cc	11	65.99%
pdns/recursordist/test-syncres_cc1.cc	13	89.73%
ext/json11/json11.cpp	14	62.65%

Totals
Change from base Build 17757454770:	0.1%
Covered Lines:	128913
Relevant Lines:	166486

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💛 - Coveralls