jpg: fix colors for sequential RGB

Fixes Ticket850

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

libavcodec/mjpegdec.c

@@ -1168,6 +1168,8 @@ int ff_mjpeg_decode_sos(MJpegDecodeContext *s, const uint8_t *mb_bitmask,
 
         if(nb_components == 3 && s->nb_components == 3 && s->avctx->pix_fmt == PIX_FMT_GBR24P)
             index = (i+2)%3;
+        if(nb_components == 1 && s->nb_components == 3 && s->avctx->pix_fmt == PIX_FMT_GBR24P)
+            index = (index+2)%3;
 
         s->comp_index[i] = index;