code review 4628045: os: Change Waitmsg.String() to operate on pointer.

src/pkg/os/exec_posix.go

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package os
 
 import (
         "runtime"
         "syscall"
 )
@@ skipping 111 matching lines @@
 
         if i < 0 {
                 bp--
                 b[bp] = '-'
         }
 
         return string(b[bp:])
 }
 
 func (w *Waitmsg) String() string {
+        if w == nil {
+                return "<nil>"
+        }
         // TODO(austin) Use signal names when possible?
         res := ""
         switch {
         case w.Exited():
                 res = "exit status " + itod(w.ExitStatus())
         case w.Signaled():
                 res = "signal " + itod(w.Signal())
         case w.Stopped():
                 res = "stop signal " + itod(w.StopSignal())
                 if w.StopSignal() == syscall.SIGTRAP && w.TrapCause() != 0 {
                         res += " (trap " + itod(w.TrapCause()) + ")"
                 }
         case w.Continued():
                 res = "continued"
         }
         if w.CoreDump() {
                 res += " (core dumped)"
         }
         return res
 }