Make udp_disconnect() clear its remote address association.

doc/rawapi.txt

@@ -272,6 +272,11 @@ level of complexity of UDP, the interface is significantly simpler.
   Sets the remote end of the pcb. This function does not generate any
   network traffic, but only set the remote address of the pcb.
 
+- err_t udp_disconnect(struct udp_pcb *pcb)
+
+  Remove the remote end of the pcb. This function does not generate
+  any network traffic, but only removes the remote address of the pcb.
+
 - err_t udp_send(struct udp_pcb *pcb, struct pbuf *p)
 
   Sends the pbuf p. The pbuf is not deallocated.

src/core/udp.c

@@ -703,6 +703,10 @@ udp_connect(struct udp_pcb *pcb, struct ip_addr *ipaddr, u16_t port)
 void
 udp_disconnect(struct udp_pcb *pcb)
 {
+  /* reset remote address association */
+  ip_addr_set(&pcb->remote_ip, IP_ADDR_ANY);
+  pcb->remote_port = 0;
+  /* mark PCB as unconnected */
   pcb->flags &= ~UDP_FLAGS_CONNECTED;
 }
 

src/include/lwip/err.h

@@ -59,6 +59,7 @@ typedef s8_t err_t;
 #define ERR_USE  -10     /* Address in use.          */
 
 #define ERR_IF   -11     /* Low-level netif error    */
+#define ERR_ISCONN -12   /* Already connected.       */
 
 
 #ifdef LWIP_DEBUG