esp32/machine_pwm: Verify PWM is active when config methods are called.

Checks are added for pwm.freq(), pwm_duty(), pwm_duty_u10() and
pwm.duty_u16().  This avoids a core dump on ESP32C3, and misleading error
messages on Xtensa ESP32 devices.

ports/esp32/machine_pwm.c

@@ -298,7 +298,14 @@ STATIC int duty_to_ns(machine_pwm_obj_t *self, int duty) {
 
 #define get_duty_raw(self) ledc_get_duty(self->mode, self->channel)
 
+STATIC void pwm_is_active(machine_pwm_obj_t *self) {
+    if (self->active == false) {
+        mp_raise_msg(&mp_type_RuntimeError, MP_ERROR_TEXT("PWM inactive"));
+    }
+}
+
 STATIC uint32_t get_duty_u16(machine_pwm_obj_t *self) {
+    pwm_is_active(self);
     int resolution = timers[TIMER_IDX(self->mode, self->timer)].duty_resolution;
     int duty = ledc_get_duty(self->mode, self->channel);
     if (resolution <= UI_RES_16_BIT) {
@@ -310,14 +317,17 @@ STATIC uint32_t get_duty_u16(machine_pwm_obj_t *self) {
 }
 
 STATIC uint32_t get_duty_u10(machine_pwm_obj_t *self) {
+    pwm_is_active(self);
     return get_duty_u16(self) >> 6; // Scale down from 16 bit to 10 bit resolution
 }
 
 STATIC uint32_t get_duty_ns(machine_pwm_obj_t *self) {
+    pwm_is_active(self);
     return duty_to_ns(self, get_duty_u16(self));
 }
 
 STATIC void set_duty_u16(machine_pwm_obj_t *self, int duty) {
+    pwm_is_active(self);
     if ((duty < 0) || (duty > UI_MAX_DUTY)) {
         mp_raise_msg_varg(&mp_type_ValueError, MP_ERROR_TEXT("duty_u16 must be from 0 to %d"), UI_MAX_DUTY);
     }
@@ -356,6 +366,7 @@ STATIC void set_duty_u16(machine_pwm_obj_t *self, int duty) {
 }
 
 STATIC void set_duty_u10(machine_pwm_obj_t *self, int duty) {
+    pwm_is_active(self);
     if ((duty < 0) || (duty > MAX_DUTY_U10)) {
         mp_raise_msg_varg(&mp_type_ValueError, MP_ERROR_TEXT("duty must be from 0 to %u"), MAX_DUTY_U10);
     }
@@ -365,6 +376,7 @@ STATIC void set_duty_u10(machine_pwm_obj_t *self, int duty) {
 }
 
 STATIC void set_duty_ns(machine_pwm_obj_t *self, int ns) {
+    pwm_is_active(self);
     if ((ns < 0) || (ns > duty_to_ns(self, UI_MAX_DUTY))) {
         mp_raise_msg_varg(&mp_type_ValueError, MP_ERROR_TEXT("duty_ns must be from 0 to %d ns"), duty_to_ns(self, UI_MAX_DUTY));
     }
@@ -598,10 +610,12 @@ STATIC void mp_machine_pwm_deinit(machine_pwm_obj_t *self) {
 // Set's and get's methods of PWM class
 
 STATIC mp_obj_t mp_machine_pwm_freq_get(machine_pwm_obj_t *self) {
+    pwm_is_active(self);
     return MP_OBJ_NEW_SMALL_INT(ledc_get_freq(self->mode, self->timer));
 }
 
 STATIC void mp_machine_pwm_freq_set(machine_pwm_obj_t *self, mp_int_t freq) {
+    pwm_is_active(self);
     if ((freq <= 0) || (freq > 40000000)) {
         mp_raise_ValueError(MP_ERROR_TEXT("freqency must be from 1Hz to 40MHz"));
     }