Fix bug #55513: Uninitialized variable in struct netconn

using patch from Karol Domagalski

(cherry picked from commit 926e39935517acc16856ddd44598b3f7815efeba)

src/api/api_msg.c

@@ -716,6 +716,9 @@ netconn_alloc(enum netconn_type t, netconn_callback callback)
   conn->pending_err = ERR_OK;
   conn->type = t;
   conn->pcb.tcp = NULL;
+#if LWIP_NETCONN_FULLDUPLEX
+  conn->mbox_threads_waiting = 0;
+#endif
 
   /* If all sizes are the same, every compiler should optimize this switch to nothing */
   switch (NETCONNTYPE_GROUP(t)) {