vss.c: Improve the fec timing code.

This patch

	- Fixes a the bug where assert(start_buf) caused para_server
	to abort.
	- Gets rid of buffer underruns by introducing a quick hack that
	sends all but the first slice a bit early.
	- Moves some code from compute_next_fec_slice() to
	setup_next_fec_group() where it belongs.