lib/ffmpeg/IOContext: allow partial reads

NEWS

 ver 0.22 (not yet released)
+* input
+  - ffmpeg: allow partial reads
 
 ver 0.21.6 (not yet released)
 * protocol

src/lib/ffmpeg/IOContext.hxx

@@ -71,7 +71,7 @@ public:
 	}
 
 	size_t Read(void *buffer, size_t size) {
-		int result = avio_read(io_context,
+		int result = avio_read_partial(io_context,
 					       (unsigned char *)buffer, size);
 		if (result < 0)
 			throw MakeFfmpegError(result, "avio_read() failed");