avcodec/mjpegdec: Fix undefined shift

Fixes CID1194388
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit b4329605)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -1229,7 +1229,7 @@ static int mjpeg_decode_scan_progressive_ac(MJpegDecodeContext *s, int ss,
     }
 
     if (!Al) {
-        s->coefs_finished[c] |= (1LL << (se + 1)) - (1LL << ss);
+        s->coefs_finished[c] |= (2LL << se) - (1LL << ss);
         last_scan = !~s->coefs_finished[c];
     }