mov: Do not allow updating the time scale after it has been set

The time scale is set in mdhd, and later validated in the
enclosing trak atom once all of its children have been parsed.

A loose mdhd atom outside of a trak atom could update the time
scale of the last stream without any validation.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
