os: call f.Close before t.Fatal to avoid vet unreachable code warnings

Change-Id: Ic3f756ecf9b4fad8fb8c259e7dad6df894863b0e
GitHub-Last-Rev: 303e524
GitHub-Pull-Request: #41900
Reviewed-on: https://go-review.googlesource.com/c/go/+/261217
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Trust: Emmanuel Odeke <[email protected]>

Author: geektutu

src/os/os_test.go

@@ -1573,8 +1573,8 @@ func TestOpenError(t *testing.T) {
 func TestOpenNoName(t *testing.T) {
 	f, err := Open("")
 	if err == nil {
-		t.Fatal(`Open("") succeeded`)
 		f.Close()
+		t.Fatal(`Open("") succeeded`)
 	}
 }