How to Alter Views Result in Drupal 7
The only way I can think is to use a views field template file to be overridden.   

Copy the template sites/all/modules/views/theme/views-view-field.tpl.php to your current enabled theme folder. Assuming that you have a block view "product-listing", you could rename it to views-view-field--product-listing--block.tpl.php.

Rescan template files in your view theming information. The file views-view-field--product-listing--block.tpl.php just created should be highlighted with bold. Now, you have an active custom field template for your block view.


Then, you can edit the template as you need. Here is a sample code:  

&lt;?php
 $alias = drupal_get_path_alias('node/'.$row-&gt;nid);
 if(drupal_valid_path($alias)){
     $url = url($alias);
?&gt;
    &lt;a href="&lt;?php print $url; ?&gt;"&gt;&lt;?php print $row-&gt;node_title; ?&gt;&lt;/a&gt;
&lt;?php
}else{
?&gt;
    &lt;span&gt;&lt;?php print $row-&gt;node_title; ?&gt;&lt;/span&gt;
&lt;?php
}


You can check if a page ( or path ) exists or not by using drupal_valid_path().