Copter: free allocations in case of allocation failure

these would be leaked if the "new" call for the ModeGuidedCustom object failed.

Since resgister_custom_mode may be called multiple times we could leak memory continuously

ArduCopter/Copter.cpp

@@ -451,6 +451,8 @@ AP_Vehicle::custom_mode_state* Copter::register_custom_mode(const uint8_t num, c
             }
             if (mode_guided_custom[i] == nullptr) {
                 // Allocation failure
+                free((void*)full_name_copy);
+                free((void*)short_name_copy);
                 return nullptr;
             }