libgsm: Set options on the right object

This fixes regressions in decoding using the libgsm_ms decoder,
broken since 0eea2129.
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavcodec/libgsm.c

@@ -155,7 +155,7 @@ static av_cold int libgsm_decode_init(AVCodecContext *avctx) {
         break;
     case CODEC_ID_GSM_MS: {
         int one = 1;
-        gsm_option(avctx->priv_data, GSM_OPT_WAV49, &one);
+        gsm_option(s->state, GSM_OPT_WAV49, &one);
         avctx->frame_size  = 2 * GSM_FRAME_SIZE;
         avctx->block_align = GSM_MS_BLOCK_SIZE;
         }