lagarith: Correctly compute hash_shift

All the values are unsigned.

Bug-Id: 907
CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/lagarithrac.c

@@ -45,7 +45,7 @@ void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length)
 
     l->range        = 0x80;
     l->low          = *l->bytestream >> 1;
-    l->hash_shift   = FFMAX(l->scale - 8, 0);
+    l->hash_shift   = FFMAX(l->scale, 8) - 8;
 
     for (i = j = 0; i < 256; i++) {
         unsigned r = i << l->hash_shift;