avcodec/hnm4video: fix palette alpha

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit 4c501bafc0)

libavcodec/hnm4video.c

@@ -375,6 +375,7 @@ static void hnm_update_palette(AVCodecContext *avctx, uint8_t *src,
             hnm->palette[writeoffset] = bytestream2_get_be24(&gb);
             if (!eight_bit_colors)
                 hnm->palette[writeoffset] <<= 2;
+            hnm->palette[writeoffset] |= (0xFFU << 24);
             count--;
             writeoffset++;
         }