[tracker/wip/carlosg/fixes-for-music] libtracker-bus: Make cursor conform to API docs

[Carlos Garnacho <carlosg src gnome org>]

commit a0f4c09c6b077ce7b2f896da572474acc1867d12
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Nov 17 11:30:01 2018 +0100

    libtracker-bus: Make cursor conform to API docs
    
    It says "NULL is returned if column is not between [0,n_columns]",
    says nothing about it being a programming error though.

 src/libtracker-bus/tracker-bus-fd-cursor.vala | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/src/libtracker-bus/tracker-bus-fd-cursor.vala b/src/libtracker-bus/tracker-bus-fd-cursor.vala
index a0e881572..d42a6133f 100644
--- a/src/libtracker-bus/tracker-bus-fd-cursor.vala
+++ b/src/libtracker-bus/tracker-bus-fd-cursor.vala
@@ -63,9 +63,14 @@ class Tracker.Bus.FDCursor : Tracker.Sparql.Cursor {
        }
 
        public override unowned string? get_string (int column, out long length = null)
-       requires (column < n_columns && data != null) {
+       requires (data != null) {
                unowned string str = null;
 
+               if (column >= n_columns) {
+                       length = 0;
+                       return null;
+               }
+
                // return null instead of empty string for unbound values
                if (types[column] == Sparql.ValueType.UNBOUND) {
                        length = 0;