Fix NULL pointer exception in audiopwmio

active_audio was set to NULL, but then used in the following set_pin()
call. because of MPU, this triggered hard fault on the stm32h7.

Author: snkYmkrct

ports/stm/common-hal/audiopwmio/PWMAudioOut.c

@@ -342,7 +342,6 @@ void common_hal_audiopwmio_pwmaudioout_stop(audiopwmio_pwmaudioout_obj_t *self)
     tim_handle.Instance->CR1 &= ~TIM_CR1_CEN;
     stm_peripherals_timer_free(tim_handle.Instance);
 
-    active_audio = NULL;
     self->stopping = false;
     self->paused = false;
 
@@ -352,6 +351,8 @@ void common_hal_audiopwmio_pwmaudioout_stop(audiopwmio_pwmaudioout_obj_t *self)
         set_pin(1, GPIO_PIN_RESET);
     }
 
+    active_audio = NULL;
+
     // Cannot free buffers here because we may be called from
     // the interrupt handler, and the heap is not reentrant.
 }