Remove pmtud struct access from outside pmtud module

Since `he_internal_pmtud_handle_probe_ack` handles (id != conn->pmtud.probe_pending_id)
we do not need to check before calling the probe pending id

src/he/msg_handlers.c

@@ -81,12 +81,9 @@ he_return_code_t he_handle_msg_pong(he_conn_t *conn, uint8_t *packet, int length
   if(id == conn->ping_pending_id) {
     // Tell the host application that we received a PONG
     he_internal_generate_event(conn, HE_EVENT_PONG);
-    return HE_SUCCESS;
-  }
-  if(id == conn->pmtud.probe_pending_id) {
+  } else {
     // Received ack of a pmtud probe
     he_internal_pmtud_handle_probe_ack(conn, id);
-    return HE_SUCCESS;
   }
 
   // Ignore the pong message

test/he/test_msg_handlers.c

@@ -206,6 +206,8 @@ void test_msg_handler_pong_pmtud_ack(void) {
 void test_msg_handler_pong_mismatch_id(void) {
   conn->ping_pending_id = 42;
 
+  he_internal_pmtud_handle_probe_ack_ExpectAndReturn(conn, 999, HE_SUCCESS);
+
   he_msg_pong_t *pong = (he_msg_pong_t *)empty_data;
   pong->id = htons(999);
   ret = he_handle_msg_pong(conn, (uint8_t *)pong, sizeof(he_msg_pong_t));