os: change Waitmsg String method to use pointer receiver

Fixes #1851.

R=rsc
CC=golang-dev
https://golang.org/cl/4628045
diff --git a/src/pkg/os/exec_plan9.go b/src/pkg/os/exec_plan9.go
index 0598adc..2590dd6 100644
--- a/src/pkg/os/exec_plan9.go
+++ b/src/pkg/os/exec_plan9.go
@@ -123,6 +123,9 @@
 	return newProcess(pid, 0), nil
 }
 
-func (w Waitmsg) String() string {
+func (w *Waitmsg) String() string {
+	if w == nil {
+		return "<nil>"
+	}
 	return "exit status: " + w.Msg
 }