Fix: Unchecked reference to TIM13 in TIM8_IRQHandler

Fixes stm32duino#585

Author: ABOSTM

cores/arduino/HardwareTimer.cpp

@@ -1192,9 +1192,11 @@ extern "C" {
     }
 
 #if defined(STM32F1xx) || defined(STM32F2xx) ||defined(STM32F4xx) || defined(STM32F7xx) || defined(STM32H7xx)
+#if defined(TIMER13_BASE)
     if (HardwareTimer_Handle[TIMER13_INDEX] != NULL) {
       HAL_TIM_IRQHandler(&HardwareTimer_Handle[TIMER13_INDEX]->handle);
     }
+#endif // TIMER13_BASE
 #endif
   }