After free()ing clientData, set it to NULL

We will change rfbClientCleanup() to free the data.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

examples/example.c

@@ -66,6 +66,7 @@ typedef struct ClientData {
 static void clientgone(rfbClientPtr cl)
 {
   free(cl->clientData);
+  cl->clientData = NULL;
 }
 
 static enum rfbNewClientAction newclient(rfbClientPtr cl)

test/encodingstest.c

@@ -172,6 +172,7 @@ static void* clientLoop(void* data) {
 	}
 	free(((clientData*)client->clientData)->display);
 	free(client->clientData);
+	client->clientData = NULL;
 	if(client->frameBuffer)
 		free(client->frameBuffer);
 	rfbClientCleanup(client);