mjpegenc_common: check for codec ID before using avctx->priv_data

When coding lossless jpeg the priv context will be pointing to LJpegEncContext
rather than MpegEncContext, which the function expects.
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/mjpegenc_common.c

@@ -91,13 +91,17 @@ static void jpeg_table_header(AVCodecContext *avctx, PutBitContext *p,
 {
     int i, j, size;
     uint8_t *ptr;
-    MpegEncContext *s = avctx->priv_data;
+    MpegEncContext *s = NULL;
+
+    /* Since avctx->priv_data will point to LJpegEncContext in this case */
+    if (avctx->codec_id != AV_CODEC_ID_LJPEG)
+        s = avctx->priv_data;
 
     if (avctx->codec_id != AV_CODEC_ID_LJPEG) {
         int matrix_count = 1 + !!memcmp(luma_intra_matrix,
                                         chroma_intra_matrix,
                                         sizeof(luma_intra_matrix[0]) * 64);
-    if (s->force_duplicated_matrix)
+    if (s && s->force_duplicated_matrix)
         matrix_count = 2;
     /* quant matrixes */
     put_marker(p, DQT);
@@ -134,7 +138,7 @@ static void jpeg_table_header(AVCodecContext *avctx, PutBitContext *p,
 
     // Only MJPEG can have a variable Huffman variable. All other
     // formats use the default Huffman table.
-    if (s->out_format == FMT_MJPEG && s->huffman == HUFFMAN_TABLE_OPTIMAL) {
+    if (s && s->huffman == HUFFMAN_TABLE_OPTIMAL) {
         size += put_huffman_table(p, 0, 0, s->mjpeg_ctx->bits_dc_luminance,
                                   s->mjpeg_ctx->val_dc_luminance);
         size += put_huffman_table(p, 0, 1, s->mjpeg_ctx->bits_dc_chrominance,