NPE from BaseJsonValidator.debug while walking JSON schema

[chrishodgins]

This was found while I was creating an issue for #1167. Changing the code to look like the example below and setting Java vmargs to -Dorg.slf4j.simpleLogger.defaultLogLevel=DEBUG was enough to generate the NPE.

        ...

        JsonSchemaFactory factory = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V7);
        JsonSchema schema = factory.getSchema(schemaContents);
        ExecutionContext context = schema.createExecutionContext();
        context.getExecutionConfig().setDebugEnabled(true);
        ValidationResult result = schema.walk(context, jsonContents, InputFormat.JSON, true);
        result.getValidationMessages().forEach(m -> System.out.println(m));

NPE:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.fasterxml.jackson.databind.JsonNode.toString()" because "node" is null
        at com.networknt.schema.BaseJsonValidator.debug(BaseJsonValidator.java:138)
        at com.networknt.schema.OneOfValidator.validate(OneOfValidator.java:71)
        at com.networknt.schema.OneOfValidator.walk(OneOfValidator.java:234)
        at com.networknt.schema.JsonSchema.walk(JsonSchema.java:1278)
        at com.networknt.schema.PropertiesValidator.walkSchema(PropertiesValidator.java:197)
        at com.networknt.schema.PropertiesValidator.validate(PropertiesValidator.java:109)
        at com.networknt.schema.PropertiesValidator.walk(PropertiesValidator.java:133)
        at com.networknt.schema.JsonSchema.walk(JsonSchema.java:1278)
        at com.networknt.schema.JsonSchema.walkAtNodeInternal(JsonSchema.java:1247)
        at com.networknt.schema.JsonSchema.walk(JsonSchema.java:1098)
...