avcodec/lagarith: fix init_get_bits() size in lag_decode_arith_plane()

untested due to lack of sample
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/lagarith.c

@@ -443,7 +443,7 @@ static int lag_decode_arith_plane(LagarithContext *l, uint8_t *dst,
             offset += 4;
         }
 
-        init_get_bits(&gb, src + offset, src_size * 8);
+        init_get_bits(&gb, src + offset, (src_size - offset) * 8);
 
         if (lag_read_prob_header(&rac, &gb) < 0)
             return -1;