Fix warnings and fatal errors when validating entities.

When validating entities that contain association properties that have
not been marshalled, there should not be any warnings or fatal errors
emitted. We can assume that any association property that was not
correctly marshalled is an error and cause validation to fail.

Refs #5257
Merge pull request #5260 from cakephp/issue-5257

Fix warnings and fatal errors when validating entities.