avcodec/mjpegdec: Check index in ljpeg_decode_yuv_scan() before using it

Fixes: 04715144ba237443010554be0d05343f/asan_heap-oob_1eafc76_1737_c685b48041a563461839e4e7ab97abb8.jpg
Fixes out of array access

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegdec.c

@@ -1093,7 +1093,10 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
                         dc = mjpeg_decode_dc(s, s->dc_index[i]);
                         if(dc == 0xFFFFF)
                             return -1;
-                        if(bits<=8){
+                        if (   h * mb_x + x >= s->width
+                            || v * mb_y + y >= s->height) {
+                            // Nothing to do
+                        } else if (bits<=8) {
                         ptr = s->picture_ptr->data[c] + (linesize * (v * mb_y + y)) + (h * mb_x + x); //FIXME optimize this crap
                         if(y==0 && toprow){
                             if(x==0 && leftcol){
@@ -1161,7 +1164,10 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
                         dc = mjpeg_decode_dc(s, s->dc_index[i]);
                         if(dc == 0xFFFFF)
                             return -1;
-                        if(bits<=8){
+                        if (   h * mb_x + x >= s->width
+                            || v * mb_y + y >= s->height) {
+                            // Nothing to do
+                        } else if (bits<=8) {
                             ptr = s->picture_ptr->data[c] +
                               (linesize * (v * mb_y + y)) +
                               (h * mb_x + x); //FIXME optimize this crap