nal: Fix read/write not pushing erroneous socket back to the stack

* Based on quartiq's minimq as of 933687c2e4
* In minimq applications, a socket is expected to be returned when `nal::TcpStack::open()` is called
  * `MqttClient::read()`/`write()` takes away the TCP socket handle (wrapped as an `Option`) from its `RefCell`, and then calls `nal::TcpStack::read()`/`write()`; if NAL returns `nb::Error`, then the MQTT client will propagate and return the error, leaving `None` behind
  * Afterwards, when `MqttClient::socket_is_connected()` gets called (e.g. while polling the interface), it will detect that the socket handle is `None`, and attempt to call `nal::TcpStack::open()`
  * Since `open()` pops a socket from the array (`unused_handles`), when implementing this NAL the socket should have been pushed back to the stack, i.e. by `close()`; this prevents any future calls of `open()` from returning `NetworkError::NoSocket` due to emptiness of the array of socket handles

src/nal.rs

@@ -236,6 +236,7 @@ where
     }
 
     fn write(&self, socket: &mut Self::TcpSocket, buffer: &[u8]) -> nb::Result<usize, Self::Error> {
+        let mut write_error = false;
         let mut non_queued_bytes = &buffer[..];
         while non_queued_bytes.len() != 0 {
             let result = {
@@ -254,9 +255,18 @@ where
                     // Process the unwritten bytes again, if any
                     non_queued_bytes = &non_queued_bytes[num_bytes..]
                 }
-                Err(_) => return Err(nb::Error::Other(NetworkError::WriteFailure)),
+                Err(_) => {
+                    write_error = true;
+                    break;
                 }
             }
+        }
+        if write_error {
+            // Close the socket to push it back to the array, for
+            // re-opening the socket in the future
+            self.close(*socket)?;
+            return Err(nb::Error::Other(NetworkError::WriteFailure))
+        }   
         Ok(buffer.len())
     }
 
@@ -268,14 +278,20 @@ where
         // Enqueue received bytes into the TCP socket buffer
         self.update()?;
         self.poll()?;
+        {
             let mut sockets = self.sockets.borrow_mut();
             let socket: &mut net::socket::TcpSocket = &mut *sockets.get(*socket);
             let result = socket.recv_slice(buffer);
             match result {
-            Ok(num_bytes) => Ok(num_bytes),
-            Err(_) => Err(nb::Error::Other(NetworkError::ReadFailure)),
+                Ok(num_bytes) => { return Ok(num_bytes) },
+                Err(_) => {},
             }
         }
+        // Close the socket to push it back to the array, for
+        // re-opening the socket in the future
+        self.close(*socket)?;
+        Err(nb::Error::Other(NetworkError::ReadFailure))
+    }
 
     fn close(&self, socket: Self::TcpSocket) -> Result<(), Self::Error> {
         let mut sockets = self.sockets.borrow_mut();