avcodec/mjpegdec: Fix decoding RGBA RCT LJPEG

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegdec.c

@@ -995,7 +995,14 @@ static int ljpeg_decode_rgb_scan(MJpegDecodeContext *s, int nb_components, int p
                 skip_bits(&s->gb, 16); /* skip RSTn */
             }
         }
-        if (s->nb_components == 4) {
+        if (s->rct && s->nb_components == 4) {
+            for (mb_x = 0; mb_x < s->mb_width; mb_x++) {
+                ptr[4*mb_x + 2] = buffer[mb_x][0] - ((buffer[mb_x][1] + buffer[mb_x][2] - 0x200) >> 2);
+                ptr[4*mb_x + 1] = buffer[mb_x][1] + ptr[4*mb_x + 2];
+                ptr[4*mb_x + 3] = buffer[mb_x][2] + ptr[4*mb_x + 2];
+                ptr[4*mb_x + 0] = buffer[mb_x][3];
+            }
+        } else if (s->nb_components == 4) {
             for(i=0; i<nb_components; i++) {
                 int c= s->comp_index[i];
                 if (s->bits <= 8) {