audio-pwm: allow freq=0 aka a pause/rest in a SONG

continue processing, but do not enable pwm units, since freq=0 wouldn't produce any sound anyway (and lead to division by zero on that occasion)

quantum/audio/audio.c

@@ -210,8 +210,6 @@ void play_note(float freq, int vol) { //NOTE: vol is unused
     }
 
     // roundrobin: shifting out old tones, keeping only uniquie ones
-    if (freq<=0)
-        return;
     // if the new frequency is already amongst the active tones, shift it to the end
     bool found = false;
     for (int i = active_tones-1; i >= 0; i--) {
@@ -284,7 +282,7 @@ bool is_playing_notes(void) { return playing_notes; }
 uint8_t audio_get_number_of_active_tones(void) { return active_tones; }
 
 float audio_get_frequency(uint8_t tone_index) {
-    if ((tone_index >= active_tones)
+    if (tone_index >= active_tones)
         return 0.0f;
     return frequencies[active_tones-tone_index-1];
 }
@@ -295,9 +293,13 @@ float audio_get_processed_frequency(uint8_t tone_index) {
 
     if (tone_index >= active_tones)
         return 0.0f;
+
     uint8_t index = active_tones - tone_index - 1;
     // new tones are appended at the end, so the most recent/current is MAX-1
 
+    if (frequencies[index] <= 0.0f)
+        return 0.0f;
+
     if (glissando) { // see voices.c
         if (frequency != 0 && frequency < frequencies[index]
             && frequency < frequencies[index] * pow(2, -440 / frequencies[index] / 12 / 2)) {

quantum/audio/audio_avr_pwm.c

@@ -133,6 +133,9 @@ void         channel_1_set_frequency(float freq) {
 
     channel_1_frequency = freq;
 
+    if (freq == 0.0f) // a pause has freq=0, no need to start the PWM (also avoids division by zero)
+        return;
+
     // set pwm period
     AUDIO1_ICRx  = (uint16_t)(((float)F_CPU) / (freq * CPU_PRESCALER));
     // and duty cycle
@@ -164,6 +167,9 @@ void         channel_2_set_frequency(float freq) {
 
     channel_2_frequency = freq;
 
+    if (freq == 0.0f) // a pause has freq=0, no need to start the PWM (also avoids division by zero)
+        return;
+
     AUDIO2_ICRx = (uint16_t)(((float)F_CPU) / (freq * CPU_PRESCALER));
     AUDIO2_OCRxy = (uint16_t)((((float)F_CPU) / (freq * CPU_PRESCALER)) * note_timbre);
 }

quantum/audio/audio_chibios_pwm.c

@@ -124,6 +124,9 @@ void channel_1_set_frequency(float freq) {
 
     channel_1_frequency = freq;
 
+    if (freq <= 0.0) //a pause/rest has freq=0
+        return;
+
     pwmcnt_t period = (pwmCFG.frequency / freq);
     pwmChangePeriod(&PWMD, period);
     pwmEnableChannel(