cmd/fix: fix cast check

Need 2-result cast so we can check the result correctly.

Fixes #23762

Change-Id: Icac3a5415156fe918988f369d6022a9a29c14089
Reviewed-on: https://go-review.googlesource.com/93078
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/fix/cftype.go

@@ -119,7 +119,7 @@ func typefix(f *ast.File, badType func(string) bool) bool {
 		if !ok {
 			return
 		}
-		t := s.X.(*ast.SelectorExpr)
+		t, ok := s.X.(*ast.SelectorExpr)
 		if !ok {
 			return
 		}