Fix `mosquitto_pub -l` not handling zero length input.

Closes #1302. Thanks to Marcus Watkins.

ChangeLog.txt

@@ -45,6 +45,7 @@ Clients:
 - Fix MQTT v5 clients not being able to specify a password without a username.
   Closes #1274.
 - Fix `mosquitto_pub -l` not handling network failures. Closes #1152.
+- Fix `mosquitto_pub -l` not handling zero length input. Closes #1302.
 - Fix double free on exit in mosquitto_pub. Closes #1280.
 
 Documentation:

client/pub_shared.c

@@ -36,7 +36,7 @@ Contributors:
 
 /* Global variables for use in callbacks. See sub_client.c for an example of
  * using a struct to hold variables for use in callbacks. */
-int mid_sent = 0;
+int mid_sent = -1;
 int status = STATUS_CONNECTING;
 struct mosq_config cfg;