Fix TypeError when 'properties' are checked against non-object

If the schema has "properties" and the incoming value is null, the validate function throws a TypeError exception:

TypeError: Cannot read property 'name' of null
      at Validator.validateProperties (/Users/afischer/jsonschema/lib/attribute.js:147:40)
      at Validator.validateSchema (/Users/afischer/jsonschema/lib/validator.js:208:34)
      at Validator.validate (/Users/afischer/jsonschema/lib/validator.js:137:23)
      at Context.it.validator.validate.name (/Users/afischer/jsonschema/test/objects.js:78:22)
      at Test.Runnable.run (/Users/afischer/jsonschema/node_modules/mocha/lib/runnable.js:213:32)
      at Runner.runTest (/Users/afischer/jsonschema/node_modules/mocha/lib/runner.js:344:10)
      at Runner.runTests.next (/Users/afischer/jsonschema/node_modules/mocha/lib/runner.js:390:12)
      at next (/Users/afischer/jsonschema/node_modules/mocha/lib/runner.js:270:14)
      at Runner.hooks (/Users/afischer/jsonschema/node_modules/mocha/lib/runner.js:279:7)
      at next (/Users/afischer/jsonschema/node_modules/mocha/lib/runner.js:227:23)
      at Runner.hook (/Users/afischer/jsonschema/node_modules/mocha/lib/runner.js:247:5)
      at process.startup.processNextTick.process._tickCallback (node.js:244:9)


From looking at the spec here ( http://tools.ietf.org/html/draft-zyp-json-schema-03#section-5.2 ), it seems like "properties" should just be ignored for a non-object instance value.

Attached is my attempt at a fix. Thanks