avcodec/mjpegdec: use the correct linesize in the flipping code

Fixes out of array access
No releases should be affected
Depends on 7c3700cd1d, do not backport without this one

Fixes: asan_heap-oob_14a37fe_9111_cov_1692584941_test4.amv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -2052,12 +2052,12 @@ the_end:
                 h = FF_CEIL_RSHIFT(h, vshift);
             }
             if(dst){
-                uint8_t *dst2 = dst + s->linesize[index]*(h-1);
+                uint8_t *dst2 = dst + s->picture_ptr->linesize[index]*(h-1);
                 for (i=0; i<h/2; i++) {
                     for (j=0; j<w; j++)
                         FFSWAP(int, dst[j], dst2[j]);
-                    dst  += s->linesize[index];
-                    dst2 -= s->linesize[index];
+                    dst  += s->picture_ptr->linesize[index];
+                    dst2 -= s->picture_ptr->linesize[index];
                 }
             }
         }