Add some safety checks to data sources.

With this change, an error is now returned when the requested frame
count is zero. Two reasons for this:

  1) It usually means there's an error in the higher level logic if
     something is trying to read zero frames.

  2) When no frames are read, MA_AT_END should be returned. However, if
     the input frame count is also zero, it creates ambiguity as to
     whether or not the data source is truly at the end.

extras/miniaudio_libopus.h

@@ -268,6 +268,14 @@ MA_API void ma_libopus_uninit(ma_libopus* pOpus, const ma_allocation_callbacks*
 
 MA_API ma_result ma_libopus_read_pcm_frames(ma_libopus* pOpus, void* pFramesOut, ma_uint64 frameCount, ma_uint64* pFramesRead)
 {
+    if (pFramesRead != NULL) {
+        *pFramesRead = 0;
+    }
+
+    if (frameCount == 0) {
+        return MA_INVALID_ARGS;
+    }
+
     if (pOpus == NULL) {
         return MA_INVALID_ARGS;
     }
@@ -317,6 +325,10 @@ MA_API ma_result ma_libopus_read_pcm_frames(ma_libopus* pOpus, void* pFramesOut,
             *pFramesRead = totalFramesRead;
         }
 
+        if (result == MA_SUCCESS && totalFramesRead == 0) {
+            result = MA_AT_END;
+        }
+
         return result;
     }
     #else