cmd/vet/internal/cfg: don't crash on malformed goto statement

Change-Id: Ib285c02e240f02e9d5511bd448163ec1d4e75516
Reviewed-on: https://go-review.googlesource.com/24323Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/internal/cfg/builder.go

@@ -98,8 +98,10 @@ start:
 			}
 
 		case token.GOTO:
+			if s.Label != nil {
 				block = b.labeledBlock(s.Label)._goto
 			}
+		}
 		if block == nil {
 			block = b.newBlock("undefined.branch")
 		}

src/cmd/vet/internal/cfg/cfg_test.go

@@ -122,6 +122,12 @@ func f10(ch chan int) {
 	}
 	live()
 }
+
+func f11() {
+	goto; // mustn't crash
+	dead()
+}
+
 `
 
 func TestDeadCode(t *testing.T) {