[udp] add Udp::IsBound() method (#3708)

src/core/meshcop/dtls.cpp

@@ -274,7 +274,7 @@ otError Dtls::Bind(TransportCallback aCallback, void *aContext)
     otError error = OT_ERROR_NONE;
 
     VerifyOrExit(mState == kStateOpen, error = OT_ERROR_INVALID_STATE);
-    VerifyOrExit(mSocket.GetSockName().mPort == 0, error = OT_ERROR_ALREADY);
+    VerifyOrExit(!mSocket.IsBound(), error = OT_ERROR_ALREADY);
     VerifyOrExit(mTransportCallback == NULL, error = OT_ERROR_ALREADY);
 
     mTransportCallback = aCallback;

src/core/net/udp6.cpp

@@ -101,7 +101,7 @@ otError UdpSocket::Bind(const SockAddr &aSockAddr)
 
     mSockName = aSockAddr;
 
-    if (mSockName.mPort == 0)
+    if (!IsBound())
     {
         do
         {
@@ -182,7 +182,7 @@ otError UdpSocket::SendTo(Message &aMessage, const MessageInfo &aMessageInfo)
         messageInfoLocal.SetSockAddr(GetSockName().GetAddress());
     }
 
-    if (GetSockName().mPort == 0)
+    if (!IsBound())
     {
         SuccessOrExit(error = Bind(GetSockName()));
     }

src/core/net/udp6.hpp

@@ -143,6 +143,15 @@ public:
      */
     otError Bind(const SockAddr &aSockAddr);
 
+    /**
+     * This method indicates whether or not the socket is bound.
+     *
+     * @retval TRUE if the socket is bound (i.e. source port is non-zero).
+     * @retval FALSE if the socket is not bound (source port is zero).
+     *
+     */
+    bool IsBound(void) const { return mSockName.mPort != 0; }
+
     /**
      * This method connects the UDP socket.
      *