Pre condition to check post is set when getting field value.

Added a pre condition to the method to ensure that $post is actually set before attempting to access the ID value of it. If it's not set then false is returned.

I am using a wrapper around the get_field method so I can couple it with an existing hard coded template variable system I have been using. I was getting the below notice when trying to access one of the variables, so I thought that I would add the pre condition.

_Notice: Trying to get property of non-object in /Users/chrispaynter/Sites/russell/src/wp-content/plugins/advanced-custom-fields/core/api.php_