Fix errors being reported during purge relationships

If the RELS-{IN,EX}T file didn't exist, we would encounter errors.  Now, we
check that there's an actual file there before attempting to act on it.

api/fedora_item.inc

@@ -389,17 +389,18 @@ RDF;
    *   Whether or not this operation has produced any changes in the RELS-EXT
    */
   function purge_relationships($relationship, $object, $namespaceURI = RELS_EXT_URI, $literal_value = FALSE) {
-    $relsext = $this->get_datastream_dissemination('RELS-EXT');
+    $relsext = trim($this->get_datastream_dissemination('RELS-EXT'));
 
-    $relsextxml = new DomDocument();
+    $relsextxml = new DOMDocument();
-    $relsextxml->loadXML($relsext);
     $modified = FALSE;
+
+    if ($relsext && $relsextxml->loadXML($relsext)) {
       $rels = $relsextxml->getElementsByTagNameNS($namespaceURI, $relationship);
       if (!empty($rels)) {
         // iterate backwards so if we delete something our pointer doesn't get out of sync
         for ($i = $rels->length; $i > 0; $i--) {
           $rel = $rels->item($i - 1);
-//      foreach ($rels as $rel) { // moving forward like this caused iteration errors when something was deleted
+  //      foreach ($rels as $rel) { // moving forward like this caused iteration errors when something was deleted
           if (//If either no object is specified, or the object matches (in either the literal or URI case), remove this node from it's parent, and mark as changed.
               empty($object) ||
               (($literal_value == RELS_TYPE_URI) && $rel->getAttributeNS('http://www.w3.org/1999/02/22-rdf-syntax-ns#', 'resource') == $object) ||
@@ -416,7 +417,7 @@ RDF;
       if ($modified) {
         $this->modify_datastream($relsextxml->saveXML(), 'RELS-EXT', "Fedora Object-to-Object Relationship Metadata", 'text/xml');
       }
-
+    }
     //Return whether or not we've introduced any changes.
     return $modified;
   }
@@ -433,11 +434,11 @@ RDF;
       $dsid = $f_prefix . $this->pid . '/' . $dsid;
     }
 
-    $relsint = $this->get_datastream_dissemination('RELS-INT');
-
     $relsxml = new DOMDocument();
-    $relsxml->loadXML($relsint);
+    $relsint = trim($this->get_datastream_dissemination('RELS-INT'));
     $modified = FALSE;
+    if ($relsint && $relsxml->loadXML($relsint)) {
+
 
       $descs = $relsxml->getElementsByTagNameNS('http://www.w3.org/1999/02/22-rdf-syntax-ns#', 'Description');
       $description = NULL;
@@ -467,11 +468,11 @@ RDF;
         }
       }
 
-
       //Save changes.
       if ($modified) {
         $this->modify_datastream($relsxml->saveXML(), 'RELS-INT', "Fedora Datastream Relationship Metadata", 'text/xml');
       }
+    }
 
     //Return whether or not we've introduced any changes.
     return $modified;