Add rule configuration input validation to commerce_payment_method_instance_load()

This could be considered bad code in another module (commerce_cardonfile), but the root cause is from commerce_payment_method_instance_load() imho.

I've been creating, disabling, re-creating a bunch of payment processors.

Commerce CardOnFile attempts to load a payment instance_id which is associated with a cardonfile entry. Unfortunately that processor is now disabled, but cardonfile assumes it's still active and calls commerce_payment_method_instance_load(). CardOnFile correctly checks to see if anything is returned from this function and if nothing is returned it handles the code correctly, but unfortunately commerce_payment_method_instance_load throws an exception as it's code expects a rule to be associated with any properly formatted call to that function.

Here's the code in question:

function commerce_payment_method_instance_load($instance_id) {
  // Return FALSE if there is no pipe delimeter in the instance ID.   
  if (strpos($instance_id, '|') === FALSE) {
    return FALSE;    
  }   

  // Explode the method key into its component parts.  
  list($method_id, $rule_name) = explode('|', $instance_id);

  // Return FALSE if we didn't receive a proper instance ID.  
  if (empty($method_id) || empty($rule_name)) { 
    return FALSE;   
  } 

  // First load the payment method and add the instance ID.   
  $payment_method = commerce_payment_method_load($method_id);  
  $payment_method['instance_id'] = $instance_id;                                                                                           
                                                                                                                                           
  // Then load the Rule configuration that enables the method.      
  // NOTE: Loading a $rule_name which doesn't exists, return null or something.                                                                       
  $rule = rules_config_load($rule_name);  

  // Iterate over its actions to find one with the matching element ID and pull 
  // its settings into the payment method object.  
  $payment_method['settings'] = array();  

  // NOTE: can't call ->actions() on NULL.
  foreach ($rule->actions() as $action) { 

So a test should be made on to see if the rule could be loaded correctly and if it can't return nothing and don't call $rule->actions()

This could be fixed in cardonfile, but I think if you call a load function with an argument that can't be loaded, it should return nothing, instead of throwing a exception.

I'll cross reference my card on file bug, so you guys can figure out where the best place to resolve this issue would be.