GCS_Common: reject negative intervals

Author: robertlong13

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -3157,6 +3157,8 @@ MAV_RESULT GCS_MAVLINK::set_message_interval(uint32_t msg_id, int32_t interval_u
     } else if (interval_us == -1) {
         // minus-one is "stop sending"
         interval_ms = 0;
+    } else if (interval_us < 0) {  
+        return MAV_RESULT_DENIED; 
     } else if (interval_us < 1000) {
         // don't squash sub-ms times to zero
         interval_ms = 1;