conncheck: Connection check reply must have a remote candidate

Ensure that a conncheck reply is coming from an address and port of a
known remote candidate and that the type of incoming socket matches that
candidate's transport.

Attemps to fix a Coverity issue in which no matching remote_candidate
gets found for a connectivity reply in conn_check_handle_inbound_stun()
(apparently due to transport mismatch), yet
priv_map_reply_to_conn_check_request() still successfully matches it
with a previous request.
4 jobs for master in 8 minutes and 50 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #1641476
1809 windows docker
build msvc amd64

00:03:53

passed #1641477
1809 windows docker
build msvc x86

00:04:24

passed #1641475
1809 windows docker
build msys2

00:07:36

 
  Test
passed #1641478
submit-to-coverity

00:01:13