dnsdist: Fix warnings from cppcheck

dnsdist: Fix warnings from cppcheck

Most of these are 'performance' warnings in parts of code where performance
does not really matter, but there is no harm in being more consistent.
It also makes it easier to spot more meaningful warnings in the future.
The remaining changes are false-positives where cppcheck does not detect
that null-pointer dereferences cannot actually happen, so I edited
the code to make it clearer (removing redundant checks in some cases,
actually adding some in other cases).