Avoid IndexError when definition titles have less than 3 components

This will cause `openapi2jsonschema to skip definitions that have
fewer than 3 components in the name (like `v1.APIGroup`), rather than
crashing with an IndexError.

Closes instrumenta#57

openapi2jsonschema/command.py

@@ -133,8 +133,12 @@ def default(output, schema, prefix, stand_alone, expanded, kubernetes, strict):
     for title in components:
         kind = title.split(".")[-1].lower()
         if kubernetes:
-            group = title.split(".")[-3].lower()
-            api_version = title.split(".")[-2].lower()
+            try:
+                group = title.split(".")[-3].lower()
+                api_version = title.split(".")[-2].lower()
+            except IndexError:
+                error(f'unable to determine group and apiversion from {title}')
+                continue
         specification = components[title]
         specification["$schema"] = "http://json-schema.org/schema#"
         specification.setdefault("type", "object")