mp4: Use para_{c,m}alloc() to allocate memory.

mp4: Use para_{c,m}alloc() to allocate memory.

With mp4ff being internal we can afford to just abort if allocations
fail (which is what the para_ wrappers do). This allows us to simplify
the code quite a bit.

The code used to cast the pointer returned from malloc(). This is
unnecessary, so drop these casts.