eval: Remove dictwatcher from watchers queue before freeing it

This fixes a use-after-free noticed by ASAN which would occur when a
dictwatcher was still active on a dictionary when the dictionary was
freed.

    fun! MakeWatch()
      let d = {'foo': 'bar'}
      call dictwatcheradd(d, 'foo', function('...'))
    endfun

Patch-by: oni-link
Closes #5930

src/nvim/eval.c

@@ -6410,8 +6410,8 @@ static void dict_free_contents(dict_T *d) {
   while (!QUEUE_EMPTY(&d->watchers)) {
     QUEUE *w = QUEUE_HEAD(&d->watchers);
     DictWatcher *watcher = dictwatcher_node_data(w);
-    dictwatcher_free(watcher);
     QUEUE_REMOVE(w);
+    dictwatcher_free(watcher);
   }
 
   hash_clear(&d->dv_hashtab);