[tracker] examples: Check for NULL cursors in class-signal example

[Carlos Garnacho <carlosg src gnome org>]

commit 9c2caec7d07628755e78790af8805e12ec2d8e9e
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Jan 28 12:55:27 2017 +0100

    examples: Check for NULL cursors in class-signal example
    
    It is unlikely to receive errors on queries there, but still this is an
    example, so better be correct there.
    
    Coverity ID: 1188420

 examples/libtracker-sparql/class-signal.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/examples/libtracker-sparql/class-signal.c b/examples/libtracker-sparql/class-signal.c
index 3ba2ab3..63fce7b 100644
--- a/examples/libtracker-sparql/class-signal.c
+++ b/examples/libtracker-sparql/class-signal.c
@@ -33,14 +33,23 @@ handle_statement (gint subject, gint predicate)
                                 subject, predicate);
        cursor = tracker_sparql_connection_query (con, query, NULL, NULL);
        g_free (query);
+
+       if (!cursor)
+               return;
+
        tracker_sparql_cursor_next (cursor, NULL, NULL);
        pred = g_strdup (tracker_sparql_cursor_get_string (cursor, 1, NULL));
        query = g_strdup_printf ("SELECT ?t { <%s> <%s> ?t }",
                                 tracker_sparql_cursor_get_string (cursor, 0, NULL),
                                 pred);
        g_object_unref (cursor);
+
        cursor = tracker_sparql_connection_query (con, query, NULL, NULL);
        g_free (query);
+
+       if (!cursor)
+               return;
+
        while (tracker_sparql_cursor_next (cursor, NULL, NULL))
                g_print ("\t%s = %s\n", pred, tracker_sparql_cursor_get_string (cursor, 0, NULL));
        g_print ("\n");