k8s.io/component-base/logs: fix usage through Go flag package

api/v1.AddFlags only supports a pflag.FlagSet. The assumption was that code
which wants to use flag.FlagSet can use VisitAll to copy the flags. That works,
with one caveat: the flag.FlagSet help implementation will call String for the
zero value to determine whether the flag has a non-default value. This
currently leads to additional warnings at the end of the -help output:

     panic calling String method on zero v1.verbosityLevelPflag for flag v: runtime error: invalid memory address or nil pointer dereference
     panic calling String method on zero v1.vmoduleConfigurationPFlag for flag vmodule: runtime error: invalid memory address or nil pointer dereference

Supporting usage of methods with the zero value is good practice anyway and
thus gets added. This then avoids these panics.

staging/src/k8s.io/component-base/logs/api/v1/pflags.go

@@ -36,6 +36,9 @@ type vmoduleConfigurationPFlag struct {
 
 // String returns the -vmodule parameter (comma-separated list of pattern=N).
 func (wrapper vmoduleConfigurationPFlag) String() string {
+	if wrapper.value == nil {
+		return ""
+	}
 	var patterns []string
 	for _, item := range *wrapper.value {
 		patterns = append(patterns, fmt.Sprintf("%s=%d", item.FilePattern, item.Verbosity))
@@ -82,10 +85,16 @@ type verbosityLevelPflag struct {
 }
 
 func (wrapper verbosityLevelPflag) String() string {
+	if wrapper.value == nil {
+		return "0"
+	}
 	return strconv.FormatInt(int64(*wrapper.value), 10)
 }
 
 func (wrapper verbosityLevelPflag) Get() interface{} {
+	if wrapper.value == nil {
+		return VerbosityLevel(0)
+	}
 	return *wrapper.value
 }