avcodec/h264: Undefined behavior (left shift of 12852653 by 8 places cannot be...

avcodec/h264: Undefined behavior (left shift of 12852653 by 8 places cannot be represented in type 'int')

Fixes: asan_heap-oob_84f75d_8_asan_heap-oob_a2a00a_341_mbc.ts

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

libavcodec/h264.c

@@ -1358,7 +1358,7 @@ static int get_avc_nalsize(H264Context *h, const uint8_t *buf,
         return -1;
 
     for (i = 0; i < h->nal_length_size; i++)
-        nalsize = (nalsize << 8) | buf[(*buf_index)++];
+        nalsize = ((unsigned)nalsize << 8) | buf[(*buf_index)++];
     if (nalsize <= 0 || nalsize > buf_size - *buf_index) {
         av_log(h->avctx, AV_LOG_ERROR,
                "AVC: nal size %d\n", nalsize);