cmd/compile: fix argument for given format verb

Follow-up to https://go-review.googlesource.com/28394.

Change-Id: Ic4147e9ae786a4de0a3454131fac03e940ae2e76
Reviewed-on: https://go-review.googlesource.com/28417Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -894,7 +894,7 @@ func (p *printer) stmtfmt(n *Node) *printer {
 		case n.List.Len() > 0:
 			// range
 			if n.List.Len() != 2 {
-				Fatalf("bad OCASE list length %d", n.List)
+				Fatalf("bad OCASE list length %d", n.List.Len())
 			}
 			p.f("case %v..%v", n.List.First(), n.List.Second())
 		default: