Merge pull request #8336 from sentinelt/socket-closed

Fix hard-fault when socket created using accept() is closed

UNITTESTS/features/netsocket/InternetSocket/test_InternetSocket.cpp

@@ -222,6 +222,7 @@ TEST_F(TestInternetSocket, getsockopt)
 TEST_F(TestInternetSocket, sigio)
 {
     callback_is_called = false;
+    socket->open((NetworkStack *)&stack);
     socket->sigio(mbed::callback(my_callback));
     socket->close(); // Trigger event;
     EXPECT_EQ(callback_is_called, true);

features/netsocket/InternetSocket.cpp

@@ -56,6 +56,10 @@ nsapi_error_t InternetSocket::close()
     _lock.lock();
 
     nsapi_error_t ret = NSAPI_ERROR_OK;
+    if (!_stack)  {
+        return ret;
+    }
+
     if (_socket) {
         _stack->socket_attach(_socket, 0, 0);
         nsapi_socket_t socket = _socket;