fix(cid/348314): free user_copy, not user

user is passed in by the caller, which we internally copy.  We should be
freeing our copy, not the caller's string.

src/nvim/os/users.c

@@ -30,7 +30,7 @@ static void add_user(garray_T *users, char *user, bool need_copy)
 
   if (user_copy == NULL || *user_copy == NUL) {
     if (need_copy) {
-      xfree(user);
+      xfree(user_copy);
     }
     return;
   }