vss: add state variable for fec client

vss: add state variable for fec client

This merges the two boolean/integer members 'error' and 'ready' into a
single state variable with the same effect:
 * the specific value of 'error' was not evaluated within vss.c,
   where error < 0 used to disable the fec_client;
 * the ready/and error states are mutually exclusive;
 * the assertion that compute_next_fec_slice() is not called when
   the fec_client is disabled has been removed, since this function is only
   called from vss_send(), which ensures that this condition holds;
 * likewise, vss_playing() holds in compute_slice_timeout() since
   this condition is tested earlier in the caller vss_compute_timeout().