Copter: fixed range check for RC channel

ArduCopter/RC_Channel.h

@@ -31,7 +31,7 @@ public:
 
     RC_Channel_Copter obj_channels[NUM_RC_CHANNELS];
     RC_Channel_Copter *channel(const uint8_t chan) override {
-        if (chan > NUM_RC_CHANNELS) {
+        if (chan >= NUM_RC_CHANNELS) {
             return nullptr;
         }
         return &obj_channels[chan];