lavf: do basic sanity checking on muxed packets

Reject packets for non-existing or attachment streams.
lavf: ignore attachment streams for interleaving purposes

Those streams should never get any packets by definition.
vp8: use a fixed-size edge emu buffer

The reason is the same as for e588615d938f8581f0d6f3771662d08cadfc00de

Based on a patch by Ronald S. Bultje <rsbultje@gmail.com>