Fix handling of short intervals with SDS011 (Fixes opendata-stuttgart…

…#426)

Just never stop the sensor if the interval is too short.
This is better than what it did so far, always stop it and
never start it (leading to no measurements)

airrohr-firmware/airrohr-firmware.ino

@@ -2798,7 +2798,8 @@ static void fetchSensorSDS(String& s) {
 
 	debug_outln_verbose(FPSTR(DBG_TXT_START_READING), FPSTR(SENSORS_SDS011));
 
-	if (msSince(starttime) < (cfg::sending_intervall_ms - (WARMUPTIME_SDS_MS + READINGTIME_SDS_MS))) {
+	if (cfg::sending_intervall_ms > (WARMUPTIME_SDS_MS + READINGTIME_SDS_MS) &&
+		msSince(starttime) < (cfg::sending_intervall_ms - (WARMUPTIME_SDS_MS + READINGTIME_SDS_MS))) {
 		if (is_SDS_running) {
 			is_SDS_running = SDS_cmd(PmSensorCmd::Stop);
 		}