AP_SerialLED: check for valid channel

libraries/AP_SerialLED/AP_SerialLED.cpp

@@ -30,8 +30,11 @@ AP_SerialLED::AP_SerialLED()
 // set number of LEDs per pin
 bool AP_SerialLED::set_num_LEDs(uint8_t chan, uint8_t num_leds)
 {
+    if (chan >= 1 && chan <= 16 && num_leds <= 32) {
         return hal.rcout->set_neopixel_num_LEDs(chan-1, num_leds);
     }
+    return false;
+}
 
 // set RGB value on mask of LEDs. chan is PWM output, 1..16
 void AP_SerialLED::set_RGB(uint8_t chan, uint32_t ledmask, uint8_t red, uint8_t green, uint8_t blue)