conncheck.c: Fix negotiation problem

When a peer reflexive candidate is discovered, it is added to the
list of connection checks, but is is not nominated by default.

This causes a problem if parent pair is already nominated and
the remote peer does not send more requests because it has already
selected a candidate. (i.e.: This is the case of WebRtc in Firefox)
2 jobs for fix-negotiation-problem in 2 minutes and 29 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #70002
build autotools

00:01:09

passed #70003
build meson

00:02:29