cmd/internal/obj: don't crash on nil in Prog.String

I can't remember just how this happened to me, but I got an unfortunate
crash with some set of cmd/compile debug options and source code.

Change-Id: Ibef6129c50b68dad0594ac439466bfbc4b32a095
Reviewed-on: https://go-review.googlesource.com/19920
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/util.go

@@ -287,6 +287,10 @@ func CConv(s uint8) string {
 }
 
 func (p *Prog) String() string {
+	if p == nil {
+		return "<nil Prog>"
+	}
+
 	if p.Ctxt == nil {
 		return "<Prog without ctxt>"
 	}