hxxx_nal: remove useless optim

This special case is now useless since p_source is always copied to p_dest,
right after in the memmove loop.

Furthermore, this caused an invalid payload size when the buffer was actually
reallocated since the payload calculation was comparing two different buffers
offset.

CVE-2020-13428

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>
(cherry picked from commit 56822f03d7a498bb22257d6075723c0e1c7f91fe)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>