Nil pointer dereference on KeyRefreshUnknownKID

[elffjs]

The README mentions that the config property KeyRefreshUnknownKID is a boolean defaulting to false, but in code the field on Config has type *bool and is referenced in exactly one place, inside of KeySet.getKey:

	jsonKey, ok = j.Keys[kid]
	j.mux.RUnlock()

	// Check if the key was present.
	if !ok {
		// Check to see if configured to refresh on unknown kid.
		if *j.Config.KeyRefreshUnknownKID {

Thus, if you don't specify a non-nil value for this property and you reach this line because you did provide a value for KeySetURL, your program will panic. I believe we've seen this in testing.

What we could do, and what seems in line with the use of other pointer-valued config properties in this file, is change the conditional to

		if j.Config.KeyRefreshUnknownKID != nil && *j.Config.KeyRefreshUnknownKID {

[MicahParks]

Please note that this snippet is due to some copied code from github.com/MicahParks/keyfunc. It's pretty old and there has since been a v1.X.X release which fixes a few bugs.

See this relevant issue: #73

[MicahParks]

This should be fixed with #129