Revert "mpeg4videodec: raise an error if sprite_trajectory.table is NULL"

Revert "mpeg4videodec: raise an error if sprite_trajectory.table is NULL"

This reverts commit 6ac0e7818399a57e4684202bac79f35b3561ad1e.

The mpeg4video parser can reach code that presumes that a certain VLC
has been initialized; yet Libav did not ensure this and Libav bug #1012
[1] is about an ensuing crash.

Instead of fixing the root cause a simple check for whether said VLC
has already been initialized was added; said check is inherently racy.

The proper fix is of course to ensure that the VLC is initialized and
commit 7c76eaeca2791261d3f4f5c98c95f44abdbd879a already ensured this,
so there was no need to merge 6ac0e7818399a57e4684202bac79f35b3561ad1e
at all. This commit therefore reverts said commit.

[1]: https://bugzilla.libav.org/show_bug.cgi?id=1012

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>