os: don't allow Process.Kill after Process.Release

This is a user error, but killing -1 kills everything, which
is a pretty bad failure mode.

Fixes #7434

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/70140043

src/pkg/os/exec_unix.go

@@ -38,6 +38,9 @@ func (p *Process) signal(sig Signal) error {
 	if p.done() {
 		return errors.New("os: process already finished")
 	}
+	if p.Pid == -1 {
+		return errors.New("os: process already released")
+	}
 	s, ok := sig.(syscall.Signal)
 	if !ok {
 		return errors.New("os: unsupported signal type")