etld/fsm.go triggers pathological compiler case

[rogpeppe]

I tried to compile it under Go 1.7rc1 and the compiler never
completed.

See golang/go#16407 - there's a suggestion
by @josharian for a possible way to work around the issue
in fsm.go code.

[zhenjl]

It seems like the latest go 1.7.1 was able to finish the compilation:

time go test

PASS
ok github.com/zhenjl/xparse/etld 0.014s

real 2m10.354s
user 1m50.362s
sys 0m2.138s

Not great since it's still 2 minutes.