Fix mosquitto_sub encoding of special characters with %j format.

Thanks to Ben Barbour.

Closes #1220.

ChangeLog.txt

@@ -50,6 +50,8 @@ Client features:
 Client fixes:
 - mosquitto_pub wouldn't always publish all messages when using `-l` and
   QoS>0. This has been fixed.
+- mosquitto_sub was incorrectly encoding special characters when using %j
+  output format. Closes #1220.
 
 
 1.5.8 - 20190228

client/sub_client_output.c

@@ -101,7 +101,7 @@ static void write_json_payload(const char *payload, int payloadlen)
 
 	for(i=0; i<payloadlen; i++){
 		if(payload[i] == '"' || payload[i] == '\\' || (payload[i] >=0 && payload[i] < 32)){
-			printf("\\u%04d", payload[i]);
+			printf("\\u%04x", payload[i]);
 		}else{
 			fputc(payload[i], stdout);
 		}