[cli] add UDP socket openness check before sending with udp send (#10206)

Problem: Executing the `udp send` command in CLI without previously
opening the example udp socket results in an unhandled null-pointer
exception.

Solution: Before anything is processed in the
`UdpExample::Process<Cmd("send")>(Arg aArgs[])` method, the UDP socket
is checked whether it's open or not. If not, the method returns with
OT_ERROR_INVALID_STATE error avoiding the null-pointer exception.

src/cli/cli_udp.cpp

@@ -256,6 +256,8 @@ template <> otError UdpExample::Process<Cmd("send")>(Arg aArgs[])
     otMessageInfo     messageInfo;
     otMessageSettings messageSettings = {mLinkSecurityEnabled, OT_MESSAGE_PRIORITY_NORMAL};
 
+    VerifyOrExit(otUdpIsOpen(GetInstancePtr(), &mSocket), error = OT_ERROR_INVALID_STATE);
+
     ClearAllBytes(messageInfo);
 
     // Possible argument formats: