mp4: Assume udta, meta and ilst are always present.

mp4: Assume udta, meta and ilst are always present.

Under normal circumstances these atoms exist or can at least be
created by other means (e.g., by running mp4tags -a foo bar.m4a).

This patch makes mp4_open_meta() fail early if at least one of the
three atoms is missing. This allows to remove the (never tested hence
probably buggy) code which creates these atoms.