cmd/compile: avoid nil pointer deref in debugging output

Found while tracking down another issue.

Change-Id: If06eee72ce3e3f45dc00b3b68670e1e4d99c87a7
Reviewed-on: https://go-review.googlesource.com/116455
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/fmt.go

@@ -1599,7 +1599,7 @@ func (n *Node) nodedump(s fmt.State, flag FmtFlag, mode fmtMode) {
 
 	case OTYPE:
 		mode.Fprintf(s, "%v %v%j type=%v", n.Op, n.Sym, n, n.Type)
-		if recur && n.Type == nil && n.Name.Param.Ntype != nil {
+		if recur && n.Type == nil && n.Name != nil && n.Name.Param != nil && n.Name.Param.Ntype != nil {
 			indent(s)
 			mode.Fprintf(s, "%v-ntype%v", n.Op, n.Name.Param.Ntype)
 		}