Fix a few compile warnings in tcp_md5.c

addons/tcp_md5/tcp_md5.c

@@ -262,7 +262,7 @@ tcp_md5_create_digest(const ip_addr_t *ip_src, const ip_addr_t *ip_dst, const st
   const size_t addr_len = IP_ADDR_RAW_SIZE(*ip_src);
 
   if (p != NULL) {
-    LWIP_ASSERT("pbuf must not point to tcp header here!", (void *)hdr != p->payload);
+    LWIP_ASSERT("pbuf must not point to tcp header here!", (const void *)hdr != p->payload);
   }
 
   /* Generate the hash, using MD5. */
@@ -319,7 +319,7 @@ tcp_md5_is_enabled_on_pcb(const struct tcp_pcb *pcb)
 {
   if (tcp_md5_extarg_id != LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID) {
     struct tcp_md5_conn_info *info = (struct tcp_md5_conn_info *)tcp_ext_arg_get(
-      (struct tcp_pcb *)pcb, tcp_md5_extarg_id);
+      LWIP_CONST_CAST(struct tcp_pcb *, pcb), tcp_md5_extarg_id);
     if (info != NULL) {
       return 1;
     }
@@ -498,7 +498,7 @@ tcp_md5_setsockopt_hook(struct lwip_sock *sock, int level, int optname, const vo
             memset(&info->remote_addr, 0, sizeof(info->remote_addr));
             if (md5->tcpm_addr.ss_family == AF_INET) {
 #if LWIP_IPV4
-              struct sockaddr_in *sin = (struct sockaddr_in *)&md5->tcpm_addr;
+              struct sockaddr_in *sin = LWIP_CONST_CAST(struct sockaddr_in *, &md5->tcpm_addr);
               memcpy(&info->remote_addr, &sin->sin_addr, sizeof(sin->sin_addr));
               IP_SET_TYPE_VAL(info->remote_addr, IPADDR_TYPE_V4);
               info->remote_port = lwip_htons(sin->sin_port);
@@ -506,7 +506,7 @@ tcp_md5_setsockopt_hook(struct lwip_sock *sock, int level, int optname, const vo
 #endif /* LWIP_IPV4 */
             } else if (md5->tcpm_addr.ss_family == AF_INET6) {
 #if LWIP_IPV6
-              struct sockaddr_in6 *sin6 = (struct sockaddr_in6 *)&md5->tcpm_addr;
+              struct sockaddr_in6 *sin6 = LWIP_CONST_CAST(struct sockaddr_in6 *, &md5->tcpm_addr);
               memcpy(&info->remote_addr, &sin6->sin6_addr, sizeof(sin6->sin6_addr));
               IP_SET_TYPE_VAL(info->remote_addr, IPADDR_TYPE_V6);
               info->remote_port = lwip_htons(sin6->sin6_port);