Fix race condition in WiFiServer

It is possible under heavy load with multithreading that a connection gets a
tcp_abort just before tcp_accept will be called.  When we abort, we clear the
this pointer, and so when we try and recover our object we crash.

Check for aborted connections (and ignore) on WiFiServer, like we do in
WiFiClient

Author: earlephilhower

libraries/WiFi/src/WiFiServer.cpp

@@ -77,8 +77,8 @@ void WiFiServer::begin(uint16_t port, uint8_t backlog) {
     }
     _listen_pcb = listen_pcb;
     _port = _listen_pcb->local_port;
-    tcp_accept(listen_pcb, &WiFiServer::_s_accept);
     tcp_arg(listen_pcb, (void*) this);
+    tcp_accept(listen_pcb, &WiFiServer::_s_accept);
 }
 
 void WiFiServer::setNoDelay(bool nodelay) {
@@ -215,9 +215,15 @@ void WiFiServer::_discard(ClientContext* client) {
 }
 
 err_t WiFiServer::_s_accept(void *arg, tcp_pcb* newpcb, err_t err) {
-    return reinterpret_cast<WiFiServer*>(arg)->_accept(newpcb, err);
+    if (arg) {
+        return reinterpret_cast<WiFiServer*>(arg)->_accept(newpcb, err);
+    } else {
+        return ERR_OK;
+    }
 }
 
 void WiFiServer::_s_discard(void* server, ClientContext* ctx) {
-    reinterpret_cast<WiFiServer*>(server)->_discard(ctx);
+    if (server) {
+        reinterpret_cast<WiFiServer*>(server)->_discard(ctx);
+    }
 }