Add sweep limits to APU square channels

src/apu.c

@@ -239,7 +239,11 @@ static inline void nes_apu_clock_sweep(nes_apu_Channel* channel,
 
     if (channel->sweep_delay == 0) {
         decrement = 0;
-        if (    (channel->reg[1] & apu_Square_Enable) &&
+        if (channel->period < 8 * nes_clock_cpu_div) {
+            channel->period = 0;
+            channel->length = 0;
+
+        } else if (    (channel->reg[1] & apu_Square_Enable) &&
                 0 != (channel->reg[1] & apu_Square_Shift)) {
             int delta = (
                     apu_channel_raw_timer_value(channel) >>
@@ -249,8 +253,10 @@ static inline void nes_apu_clock_sweep(nes_apu_Channel* channel,
                 delta = adjust - delta;
             }
             channel->period += delta * nes_clock_apu_div;
-            if (channel->period < 0) {
+            if ( channel < 0 ||
+                 channel->period > 0x7FFU * nes_clock_cpu_div) {
                 channel->period = 0;
+                channel->length = 0;
             }
         }
     }