[udp] clear all properties in SocketHandle from Udp::Open() (#10380)

This commit modifies `Udp::Open()` to clear all properties of the
passed-in `aSocket` using the `Clear()` method. This replaces the
existing code, which only cleared the socket and peer address and
port numbers. This change ensures that `void *mHandle` member
variable in `otUdpSocket` is also set to `nullptr` before invoking
platform UDP `otPlatUdpSocket()`.

src/core/net/udp6.cpp

@@ -175,8 +175,7 @@ Error Udp::Open(SocketHandle &aSocket, otUdpReceive aHandler, void *aContext)
 
     OT_ASSERT(!IsOpen(aSocket));
 
-    aSocket.GetSockName().Clear();
-    aSocket.GetPeerName().Clear();
+    aSocket.Clear();
     aSocket.mHandler = aHandler;
     aSocket.mContext = aContext;