agent: reject addition of peer-reflexive remote candidates

Peer-reflexive candidates are not supposed to be set by the application,
since they are not exchanged in the SDP.
7 jobs for master in 16 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6171010
build

00:00:28

passed #6171012
docker windows 1809
build msvc amd64

00:02:46

passed #6171013
docker windows 1809
build msvc x86

00:02:42

passed #6171011
docker windows 1809
build msys2

00:16:52

 
  Test
passed #6171016
doc-and-install

00:00:47

passed #6171014
test

00:00:56

passed #6171015
test valgrind

00:01:01