lwip: Check for accepted connection being NULL

This happens when lwip runs out of pcbs and just communicates an error.

mongoose.c

@@ -15308,6 +15308,10 @@ static err_t mg_lwip_accept_cb(void *arg, struct tcp_pcb *newtpcb, err_t err) {
   struct mg_connection *lc = (struct mg_connection *) arg, *nc;
   struct mg_lwip_conn_state *lcs, *cs;
   struct tcp_pcb_listen *lpcb;
+  if (newtpcb == NULL) {
+    LOG(LL_ERROR, ("%p accept error %d", lc, (int) err));
+    return ERR_ABRT;
+  }
   LOG(LL_DEBUG,
       ("%p conn %p from %s:%u", lc, newtpcb,
        IPADDR_NTOA(ipX_2_ip(&newtpcb->remote_ip)), newtpcb->remote_port));

src/common/platforms/lwip/mg_lwip_net_if.c

@@ -383,6 +383,10 @@ static err_t mg_lwip_accept_cb(void *arg, struct tcp_pcb *newtpcb, err_t err) {
   struct mg_connection *lc = (struct mg_connection *) arg, *nc;
   struct mg_lwip_conn_state *lcs, *cs;
   struct tcp_pcb_listen *lpcb;
+  if (newtpcb == NULL) {
+    LOG(LL_ERROR, ("%p accept error %d", lc, (int) err));
+    return ERR_ABRT;
+  }
   LOG(LL_DEBUG,
       ("%p conn %p from %s:%u", lc, newtpcb,
        IPADDR_NTOA(ipX_2_ip(&newtpcb->remote_ip)), newtpcb->remote_port));