mft: use the size of the audio buffer to read the indexed buffer

In case there is more than one we should not read all of them each time.

This is currently a buffer we provide so it should not happen.

modules/codec/mft.cpp

@@ -817,7 +817,7 @@ static int ProcessOutputStream(decoder_t *p_dec, DWORD stream_id, bool & keep_re
                 goto error;
 
             DWORD total_length = 0;
-            hr = output_sample->GetTotalLength(&total_length);
+            hr = output_media_buffer->GetCurrentLength(&total_length);
             if (FAILED(hr))
                 goto error;