lagarithrac: fix length used in ff_lag_rac_init()

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

libavcodec/lagarithrac.c

@@ -40,8 +40,7 @@ void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length)
     align_get_bits(gb);
     l->bytestream_start =
     l->bytestream       = gb->buffer + get_bits_count(gb) / 8;
-    l->bytestream_end   = l->bytestream_start + length;
-
+    l->bytestream_end   = l->bytestream_start + get_bits_left(gb) / 8;
     l->range        = 0x80;
     l->low          = *l->bytestream >> 1;
     l->hash_shift   = FFMAX(l->scale - 8, 0);