Checking maxProperties when payload is not an object generates PHP Warning

[mirfilip]

• package version used: 2.0.1 (9ef71fd)
• schema

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "foo": {
      "type": "object",
      "maxProperties": 1,
      "additionalProperties": false,
      "patternProperties": {
        "^[a-zA-Z]+$": {
          "type": "boolean"
        }
      }
    }
  }
}

• input:

{
  "foo": 123
}

It generates:

PHP Warning:  get_object_vars() expects parameter 1 to be object, integer given in /tmp/repro/vendor/justinrainbow/json-schema/src/JsonSchema/Constraints/ObjectConstraint.php on line 169

I think it shouldn't attempt to do anything with ObjectConstraint when the payload is not an object.

For comparison, version 1.6.1 (cc84765) produces no warning (and validates properly).