avformat/riffenc: move MP3 LSF threshold to the midway point between the 2

No testcase available but this seems more correct
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/riffenc.c

@@ -105,7 +105,7 @@ int ff_put_wav_header(AVIOContext *pb, AVCodecContext *enc, int flags)
     if (enc->codec_id == AV_CODEC_ID_MP2) {
         blkalign = frame_size;
     } else if (enc->codec_id == AV_CODEC_ID_MP3) {
-        blkalign = 576 * (enc->sample_rate <= 24000 ? 1 : 2);
+        blkalign = 576 * (enc->sample_rate <= (24000 + 32000)/2 ? 1 : 2);
     } else if (enc->codec_id == AV_CODEC_ID_AC3) {
         blkalign = 3840;                /* maximum bytes per frame */
     } else if (enc->codec_id == AV_CODEC_ID_AAC) {