libgsm: Reset the MS mode of GSM in the flush function

The mode is set in libgsm_decode_init, but the decoder
object is simply destroyed and recreated in the flush
function - therefore the mode has to be set again.

This fixes playback using the libgsm_ms decoder in avplay.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 1b35af3225)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libgsm.c

@@ -212,9 +212,12 @@ static int libgsm_decode_frame(AVCodecContext *avctx, void *data,
 
 static void libgsm_flush(AVCodecContext *avctx) {
     LibGSMDecodeContext *s = avctx->priv_data;
+    int one = 1;
 
     gsm_destroy(s->state);
     s->state = gsm_create();
+    if (avctx->codec_id == CODEC_ID_GSM_MS)
+        gsm_option(s->state, GSM_OPT_WAV49, &one);
 }
 
 AVCodec ff_libgsm_decoder = {