AP_BLHeli: make ESC debug easier to see

libraries/AP_BLHeli/AP_BLHeli.cpp

@@ -1494,10 +1494,12 @@ void AP_BLHeli::log_bidir_telemetry(void)
                 trpm = trpm * 200 / motor_poles;
             }
 
+            if (trpm > 0) {
                 last_log_ms[last_telem_esc] = now;
                 DEV_PRINTF("ESC[%u] RPM=%u e=%.1f t=%u\n", last_telem_esc, trpm, hal.rcout->get_erpm_error_rate(motor_idx), (unsigned)AP_HAL::millis());
             }
         }
+    }
 
     if (!SRV_Channels::have_digital_outputs()) {
         return;