Avoid concurrent reset of NAT timer (fixes #3337)

[calmh]

Trivial little thing. Not bothering to test it because it's painful for other reasons (needing a working *config.Wrapper for example).

[AudriusButkevicius]

@st-review merge

[st-review]

@AudriusButkevicius: Merge failed:

$ git fetch -f origin refs/pull/3341/head:pr-3341
$ git fetch -f origin master:orig/master
From github.com:syncthing/syncthing
   502bee9..f343c8b  master     -> orig/master
$ git reset --hard
HEAD is now at f343c8b lib/model, lib/scanner: Silence vet warnings
$ git checkout master
Already on 'master'
Your branch is up-to-date with 'origin/master'.
$ git reset --hard orig/master
HEAD is now at f343c8b lib/model, lib/scanner: Silence vet warnings
$ git clean -fxd
Skipping repository syncthing/syncthing
Skipping repository syncthing/syncthing-android
Skipping repository syncthing/docs
Skipping repository syncthing/discosrv
Skipping repository syncthing/syncthing-gtk
Skipping repository syncthing/syncthing-inotify
$ git merge --squash --no-commit pr-3341
Updating f343c8b..1c200b6
Fast-forward
Squash commit -- not updating HEAD
 lib/nat/service.go | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
$ git commit -F -
******************************************************
* Commit message does not match the expected format: *
*                                                    *
*   some/tag: A longer description (fixes #1234)     *
*                                                    *
******************************************************

[calmh]

Sorry.

[calmh]

@st-review merge it

lib/nat: Avoid concurrent reset of NAT timer (fixes #3337)

[st-review]

👌 Merged as a2dcffc. Thanks, @calmh!