internal/obj: protect against nil addr.Sym

This has been the root cause of a number of crashes caused by
fuzz throwing modem noise at the assembler, which in turn attempts
to print diagnostics but instead just gets crashes.

Fixes #12627.

Change-Id: I72c2da79d8eb240e1a37aa6140454c552b05e0f1
Reviewed-on: https://go-review.googlesource.com/14595Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/util.go

@@ -457,13 +457,25 @@ func Mconv(a *Addr) string {
 		}
 
 	case NAME_EXTERN:
+		if a.Sym != nil {
 			str = fmt.Sprintf("%s%s(SB)", a.Sym.Name, offConv(a.Offset))
+		} else {
+			str = fmt.Sprintf("%s(SB)", offConv(a.Offset))
+		}
 
 	case NAME_GOTREF:
+		if a.Sym != nil {
 			str = fmt.Sprintf("%s%s@GOT(SB)", a.Sym.Name, offConv(a.Offset))
+		} else {
+			str = fmt.Sprintf("%s@GOT(SB)", offConv(a.Offset))
+		}
 
 	case NAME_STATIC:
+		if a.Sym != nil {
 			str = fmt.Sprintf("%s<>%s(SB)", a.Sym.Name, offConv(a.Offset))
+		} else {
+			str = fmt.Sprintf("<>%s(SB)", offConv(a.Offset))
+		}
 
 	case NAME_AUTO:
 		if a.Sym != nil {