Fix panic when using default-initialized FlagSet

Closes #13294

Signed-off-by: Sebastien Rannou <mxs@sbrk.org>

pkg/mflag/flag.go

@@ -560,7 +560,7 @@ func defaultUsage(f *FlagSet) {
 // Usage prints to standard error a usage message documenting all defined command-line flags.
 // The function is a variable that may be changed to point to a custom function.
 var Usage = func() {
-	fmt.Fprintf(CommandLine.output, "Usage of %s:\n", os.Args[0])
+	fmt.Fprintf(CommandLine.Out(), "Usage of %s:\n", os.Args[0])
 	PrintDefaults()
 }