In the worst case the startcode prefix has 4 bytes.

This fixes a trigerred assertion:
Assertion dp <= max_size failed at libavcodec/cbs_h2645.c:1451

Found-by:libFuzzer
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>