[tracker] libtracker-data: Fix compiler warning

[Carlos Garnacho <carlosg src gnome org>]

commit 3b6ed2f78ee22e441d417b5d8b225dde65e4f73c
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Sep 9 23:59:22 2017 +0200

    libtracker-data: Fix compiler warning
    
    Use hashtables with nullable integer return value the vala way, instead
    of using lookup_extended(), which both looks out of place and produces
    constness related warnings on gcc.

 src/libtracker-data/tracker-sparql-query.vala |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql-query.vala b/src/libtracker-data/tracker-sparql-query.vala
index f6c1249..54ce570 100644
--- a/src/libtracker-data/tracker-sparql-query.vala
+++ b/src/libtracker-data/tracker-sparql-query.vala
@@ -176,18 +176,18 @@ namespace Tracker.Sparql {
        }
 
        class Solution {
-               public HashTable<string,int> hash;
+               public HashTable<string,int?> hash;
                public GenericArray<string> values;
                public int solution_index;
 
                public Solution () {
-                       this.hash = new HashTable<string,int> (str_hash, str_equal);
+                       this.hash = new HashTable<string,int?> (str_hash, str_equal);
                        this.values = new GenericArray<string> ();
                }
 
                public string? lookup (string variable_name) {
-                       int variable_index;
-                       if (!hash.lookup_extended (variable_name, null, out variable_index)) {
+                       int? variable_index = hash.get (variable_name);
+                       if (variable_index == null) {
                                return null;
                        }
                        return values[solution_index * hash.size () + variable_index];