Remove usage of fatalx() in here. This code will be shared with bgpctl.

Remove usage of fatalx() in here. This code will be shared with bgpctl.
In flowspec_cmp() make sure a deterministic sort is possible. Most error
cases are unreachable if flowspec NLRI are validated first (flowspec_valid).
In flowspec_valid() replace the assert like check with an error return.
OK tb@