Handle zero-frame input callbacks and output buffer size mismatches

CoreAudio can call the input callback with 0 frames during device
transitions.  The output callback already handles this case (added
for the same reason), but the input callback had an assert that
crashed the process (~21 crash pings per 30 days on macOS).  Add
the same early-return pattern.

Similarly, during device format changes there can be a transient
mismatch between the output buffer size provided by CoreAudio
and the format described by output_dev_desc.  The mixer assert
would crash if the buffer was too small (~19 crash pings per 30
days).  Replace with a check that outputs silence and returns
early, avoiding an out-of-bounds write in the mixer.

Author: kinetiknz

src/backend/mod.rs

@@ -515,12 +515,19 @@ extern "C" fn audiounit_input_callback(
         Reinit,
     }
 
-    assert!(input_frames > 0);
     assert_eq!(bus, AU_IN_BUS);
 
     assert!(!user_ptr.is_null());
     let stm = unsafe { &mut *(user_ptr as *mut AudioUnitStream) };
 
+    if input_frames == 0 {
+        cubeb_alog!(
+            "({:p}) input callback empty.",
+            stm as *const AudioUnitStream
+        );
+        return NO_ERR;
+    }
+
     if unsafe { *flags | kAudioTimeStampHostTimeValid } != 0 {
         let now = unsafe { mach_absolute_time() };
         let input_latency_frames = compute_input_latency(stm, unsafe { (*tstamp).mHostTime }, now);
@@ -980,10 +987,17 @@ extern "C" fn audiounit_output_callback(
 
     // Mixing
     if let Some(mixer) = stm.core_stream_data.mixer.as_mut() {
-        assert!(
-            buffers[0].mDataByteSize
-                >= stm.core_stream_data.output_dev_desc.mBytesPerFrame * output_frames
-        );
+        let needed = stm.core_stream_data.output_dev_desc.mBytesPerFrame * output_frames;
+        if buffers[0].mDataByteSize < needed {
+            cubeb_log!(
+                "({:p}) output buffer too small for mixer: have {} bytes, need {} bytes",
+                stm as *const AudioUnitStream,
+                buffers[0].mDataByteSize,
+                needed
+            );
+            audiounit_make_silent(&buffers[0]);
+            return NO_ERR;
+        }
         mixer.mix(
             output_frames as usize,
             buffers[0].mData,