lib: Fix a crash when applying a schema with empty items
lib: Fix a crash when applying a schema with empty items

The schema
   { "items": [] }
caused a crash due to trying to dereference a NULL additionalItems
subschema, when applying the overall schema.

This is the first bug Walbottle has found in itself through self-hosted
test vector generation over the JSON Schema meta-schema. 🎉

pwithnall/walbottle