conncheck: Don't consider unmatched STUN response as media

Previously, a STUN response for which there was no associated request
would be considered valid media input and as such could keep a dead
connection alive. If peer A was communicating with peer B and peer B got
disconnected, the keepalive mechanism in peer A should detect this.
However, a misbehaving STUN server could keep sending STUN responses to
peer A which would then be considered a valid communication between A
and B and thereby prevent the keepalive mechanism from shutting down the
connection.
8 jobs for unmatched_responses in 5 minutes and 40 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #3543361
build

00:00:24

passed #3543363
1809 windows docker
build msvc amd64

00:04:12

passed #3543364
1809 windows docker
build msvc x86

00:04:17

failed #3551141
1809 windows docker allowed to fail
build msys2

00:01:19

failed #3543362
1809 windows docker allowed to fail
build msys2

00:02:54

 
  Test
passed #3543367
doc-and-install

00:00:53

passed #3543365
test

00:01:07

failed #3543366
test valgrind

00:00:54

 
Name Stage Failure
failed
build msys2 Build
Git branch:   13-1-stable
GO version: go1.13.8
Built: 2020-06-19T20:36:38+0000
OS/Arch: windows/amd64
Uploading artifacts...
WARNING: build/meson-logs/: no matching files
WARNING: build/build.ninja: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 3221225477
failed
test valgrind Test
==743== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 22 from 22)
-------

Full log written to /builds/johast/libnice/build/meson-logs/testlog-valgrind.txt
Uploading artifacts for failed job
Uploading artifacts...
build/meson-logs/: found 4 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=3543366 responseStatus=201 Created token=SEV1x7ug
ERROR: Job failed: exit code 1