libgsmdec: check output buffer size before decoding

(cherry picked from commit b03761b130)

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

libavcodec/libgsm.c

@@ -141,18 +141,25 @@ static int libgsm_decode_frame(AVCodecContext *avctx,
                                AVPacket *avpkt) {
     const uint8_t *buf = avpkt->data;
     int buf_size = avpkt->size;
+    int out_size = avctx->frame_size * av_get_bytes_per_sample(avctx->sample_fmt);
+
+    if (*data_size < out_size) {
+        av_log(avctx, AV_LOG_ERROR, "Output buffer is too small\n");
+        return AVERROR(EINVAL);
+    }
+
     *data_size = 0; /* In case of error */
     if(buf_size < avctx->block_align) return -1;
     switch(avctx->codec_id) {
     case CODEC_ID_GSM:
         if(gsm_decode(avctx->priv_data,buf,data)) return -1;
-        *data_size = GSM_FRAME_SIZE*sizeof(int16_t);
         break;
     case CODEC_ID_GSM_MS:
         if(gsm_decode(avctx->priv_data,buf,data) ||
            gsm_decode(avctx->priv_data,buf+33,((int16_t*)data)+GSM_FRAME_SIZE)) return -1;
-        *data_size = GSM_FRAME_SIZE*sizeof(int16_t)*2;
     }
+
+    *data_size = out_size;
     return avctx->block_align;
 }