Bug fix: never create row w/ session ID but no search data.

- There was a race condition where multiple parallel requests could end up with a fatal error caused by one process retrieving the partially created row generated by another process and trying to unserialize an empty string. By waiting to write the session ID to the database until we also have the data available, we can avoid this problem.

module/VuFind/src/VuFind/Db/Table/Search.php

@@ -181,15 +181,16 @@ class Search extends Gateway
 
         // If we got this far, we didn't find a saved duplicate, so we should
         // save the new search:
-        $data = [
-            'session_id' => $sessionId,
-            'created' => date('Y-m-d'),
-        ];
-        $this->insert($data);
+        $this->insert(['created' => date('Y-m-d')]);
         $row = $this->getRowById($this->getLastInsertValue());
 
         // Chicken and egg... We didn't know the id before insert
         $newSearch->updateSaveStatus($row);
+
+        // Don't set session ID until this stage, because we don't want to risk
+        // ever having a row that's associated with a session but which has no
+        // search object data attached to it; this could cause problems!
+        $row->session_id = $sessionId;
         $row->search_object = serialize(new minSO($newSearch));
         $row->save();
     }