avcodec/lagarith: fix integer overflow

Fixes: asan_heap-oob_1bf48fa_2513_lag-yuy2.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/lagarith.c

@@ -128,7 +128,7 @@ static int lag_decode_prob(GetBitContext *gb, uint32_t *value)
     }
 
     val  = get_bits_long(gb, bits);
-    val |= 1 << bits;
+    val |= 1U << bits;
 
     *value = val - 1;