clang-tidy: DynListener::getLine.

clang-tidy: DynListener::getLine.

Fix founds with:

- [readability-implicit-bool-conversion]
  - pdns/dynlistener.cc:256
  - pdns/dynlistener.cc:272:8

- [readability-container-data-pointer]
  - pdns/dynlistener.cc:240:19
  - pdns/dynlistener.cc:245:25
  - pdns/dynlistener.cc:256:17
  - dns/dynlistener.cc:263:17

- [bugprone-narrowing-conversions,cppcoreguidelines-narrowing-conversions]
  - pdns/dynlistener.cc:240:29
  - pdns/dynlistener.cc:256:27

Avoided to fix `if` related to avoid making too much noise.