h2645_parse: use the bytestream2 API for packet splitting

The code does some nontrivial jumping around in the buffer, so it is
safer to use a checked API rather than do everything manually.

Fixes a bug in nalff parsing, where the length field is currently not
counted in the buffer size check, resulting in possible overreads with
invalid files.

CC: libav-stable@libav.org
Bug-Id: 1002
Found-By: Kamil Frankowicz
