[329] Fix potential negative timeout being passed to pselect.

Thanks to Dollars.

Bug: https://github.com/eclipse/mosquitto/issues/329

ChangeLog.txt

@@ -6,6 +6,7 @@ Broker:
 Client library:
 - Clients can now use TLS with IPv6.
 - Fix potential socket leakage when reconnecting. Closes #304.
+- Fix potential negative timeout being passed to pselect. Closes #329.
 
 
 1.4.10 - 20160816

lib/mosquitto.c

@@ -916,6 +916,12 @@ int mosquitto_loop(struct mosquitto *mosq, int timeout, int max_packets)
 		timeout = (mosq->next_msg_out - now)*1000;
 	}
 
+	if(timeout < 0){
+		/* There has been a delay somewhere which means we should have already
+		 * sent a message. */
+		timeout = 0;
+	}
+
 	local_timeout.tv_sec = timeout/1000;
 #ifdef HAVE_PSELECT
 	local_timeout.tv_nsec = (timeout-local_timeout.tv_sec*1000)*1e6;