shorten: Fix out of bound writes in fix_bitshift()

The data pointers s->decoded[*] already take into account s->nwrap.
Signed-off-by: Janne Grunau <janne-libav@jannau.net>

libavcodec/shorten.c

@@ -155,7 +155,7 @@ static void fix_bitshift(ShortenContext *s, int32_t *buffer)
 
     if (s->bitshift != 0)
         for (i = 0; i < s->blocksize; i++)
-            buffer[s->nwrap + i] <<= s->bitshift;
+            buffer[i] <<= s->bitshift;
 }