lib: fix build error and compiler warnings with verbose strings disabled

- asyn-ares: fix compiler warning:
  ```
  lib/asyn-ares.c:751:17: error: code will never be executed [clang-diagnostic-unreachable-code,-warnings-as-errors]
    751 |     char *csv = ares_get_servers_csv(ares->channel);
        |                 ^~~~~~~~~~~~~~~~~~~~
  ```

- curl_trc: fix missing symbol:
  ```
  /usr/bin/ld: ../lib/.libs/libcurl.so: undefined reference to `Curl_trc_timer'
  collect2: error: ld returned 1 exit status
  ```
  Ref: https://app.circleci.com/pipelines/github/curl/curl/15446/workflows/67afa113-9c49-4249-9180-f6f01fc7dfdd/jobs/149177
  Ref: https://github.com/curl/curl/actions/runs/18174250400/job/51736249444#step:33:623
  Follow-up to b022389757 #18768

- multi: fix `-Wunreachable-code`:
  ```
  lib/multi.c:1107:28: error: code will never be executed [-Werror,-Wunreachable-code]
   1107 |     size_t timeout_count = Curl_llist_count(&data->state.timeoutlist);
        |                            ^~~~~~~~~~~~~~~~
  lib/multi.c:3054:35: error: code will never be executed [-Werror,-Wunreachable-code]
   3054 |       struct Curl_llist_node *e = Curl_llist_head(&data->state.timeoutlist);
        |                                   ^~~~~~~~~~~~~~~
  lib/multi.c:3380:7: error: code will never be executed [-Werror,-Wunreachable-code]
   3380 |       Curl_llist_head(&data->state.timeoutlist);
        |       ^~~~~~~~~~~~~~~
  ```

Cherry-picked from #18797
Closes #18799

lib/curl_trc.c

@@ -672,6 +672,11 @@ void Curl_trc_dns(struct Curl_easy *data, const char *fmt, ...)
   (void)data; (void)fmt;
 }
 
+void Curl_trc_timer(struct Curl_easy *data, int tid, const char *fmt, ...)
+{
+  (void)data; (void)tid; (void)fmt;
+}
+
 void Curl_trc_read(struct Curl_easy *data, const char *fmt, ...)
 {
   (void)data; (void)fmt;