h264: Fix a possible overread in decode_nal_units()

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/h264.c

@@ -3764,7 +3764,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
         int err;
 
         if(buf_index >= next_avc) {
-            if(buf_index >= buf_size) break;
+            if (buf_index >= buf_size - h->nal_length_size) break;
             nalsize = 0;
             for(i = 0; i < h->nal_length_size; i++)
                 nalsize = (nalsize << 8) | buf[buf_index++];