cmd/compile: exit early if oldname doesn't find a name

Change-Id: I59ce1b40bc3472cc92a9a0a9fbd32342e0b945ad
Reviewed-on: https://go-review.googlesource.com/20152Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -388,6 +388,7 @@ func oldname(s *Sym) *Node {
 		n = newname(s)
 		n.Op = ONONAME
 		n.Name.Iota = iota_ // save current iota value in const declarations
+		return n
 	}
 
 	if Curfn != nil && n.Op == ONAME && n.Name.Funcdepth > 0 && n.Name.Funcdepth != Funcdepth {