h264: avoid stuck buffer pointer in decode_nal_units

When decode_nal_units() previously encountered a NAL_END_SEQUENCE,
and there are some junk bytes left in the input buffer, but no start codes,
buf_index gets stuck 3 bytes before the end of the buffer.

This can trigger an infinite loop in the caller code, eg. in
try_decode_trame(), as avcodec_decode_video() then keeps returning zeroes,
with 3 bytes of the input packet still available.

With this change, the remaining bytes are skipped so the whole packet gets
consumed.

CC:libav-stable@libav.org

Signed-off-by: Jindřich Makovička <makovick@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/h264.c

@@ -3694,8 +3694,10 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size)
                         buf[buf_index + 2] == 1)
                         break;
 
-                if (buf_index + 3 >= buf_size)
+                if (buf_index + 3 >= buf_size) {
+                    buf_index = buf_size;
                     break;
+                }
 
                 buf_index += 3;
                 if (buf_index >= next_avc)