diracdec: avoid overflow of bytes*8 in decode_lowdelay

If bytes is large enough, bytes*8 can overflow and become negative.

In that case 'bufsize -= bytes*8' causes bufsize to increase instead of
decrease.

This leads to a segmentation fault.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 9e66b39aa8)

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

libavcodec/diracdec.c

@@ -799,7 +799,10 @@ static void decode_lowdelay(DiracContext *s)
             slice_num++;
 
             buf     += bytes;
-            bufsize -= bytes*8;
+            if (bufsize/8 >= bytes)
+                bufsize -= bytes*8;
+            else
+                bufsize = 0;
         }
 
     avctx->execute(avctx, decode_lowdelay_slice, slices, NULL, slice_num,