Crash when union in exceptions.json not previously defined

[bbredesen]

Found when running against 1.2.177, exceptions.json has a union type defined (VkAccelerationStructureMotionInstanceDataNV), which does not exist in that version of vk.xml.

Crash is here

func ReadUnionExceptionsFromJSON(exceptions gjson.Result, tr TypeRegistry, vr ValueRegistry) {
	exceptions.Get("union").ForEach(func(key, exVal gjson.Result) bool {
		if key.String() == "!comment" {
			return true
		} // Ignore comments

		UpdateUnionTypeFromJSON(key, exVal, tr[key.String()].(*unionType)) // <-- Crash here
                 // This type coercion fail because the map lookup returns nil

		return true
	})
}