cmd/go/internal/modfetch: do not panic when zip is incorrect from GOPROXY

If GOPROXY sends the wrong zip data when cmd/go asks for a module's zip ball,
we should not panic. zip.OpenReader always returns a nil ReadCLoser if err is not nil.

Therefore, it's Okay to remove zip.Close in the err block.

Fixes #26536

src/cmd/go/internal/modfetch/fetch.go

@@ -90,7 +90,6 @@ func downloadZip(mod module.Version, target string) error {
 	// Double-check zip file looks OK.
 	z, err := zip.OpenReader(tmpfile)
 	if err != nil {
-		z.Close()
 		return err
 	}
 	prefix := mod.Path + "@" + mod.Version