Fix segfault on infinite recursion into circular model relations

In cases where related models were set before save on *both* sides of a
hasMany/belongsTo relationship, `Model::save()` would go into an infinite
loop as its pre and post-save checks for relations to save never checked
if the related model had already been saved:

    Save A
     \_ A hasMany B
      \_ Save B
       \_ B belongsTo A
        \_ Save A
         \_ ...

For me this was causing a segfault, though with xdebug installed it would
error before crashing at the max_nesting_level.

phalcon/mvc/model.zep

@@ -2793,8 +2793,9 @@ abstract class Model implements EntityInterface, ModelInterface, ResultInterface
 
 					/**
 					 * If dynamic update is enabled, saving the record must not take any action
+					 * Only save if the model is dirty to prevent circular relations causing an infinite loop
 					 */
-					if !record->save() {
+					if record->_dirtyState != Model::DIRTY_STATE_PERSISTENT && !record->save() {
 
 						/**
 						 * Get the validation messages generated by the referenced model