input: register title change callbacks only once

Fixes skipping multiple titles with single click.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/input/var.c

@@ -256,16 +256,20 @@ void input_ControlVarNavigation( input_thread_t *p_input )
     /* Create more command variables */
     if( p_input->p->i_title > 1 )
     {
+        if( var_Type( p_input, "next-title" ) == 0 ) {
             var_Create( p_input, "next-title", VLC_VAR_VOID );
             text.psz_string = _("Next title");
             var_Change( p_input, "next-title", VLC_VAR_SETTEXT, &text, NULL );
             var_AddCallback( p_input, "next-title", TitleCallback, NULL );
+        }
 
+        if( var_Type( p_input, "prev-title" ) == 0 ) {
             var_Create( p_input, "prev-title", VLC_VAR_VOID );
             text.psz_string = _("Previous title");
             var_Change( p_input, "prev-title", VLC_VAR_SETTEXT, &text, NULL );
             var_AddCallback( p_input, "prev-title", TitleCallback, NULL );
         }
+    }
 
     /* Create titles and chapters */
     var_Change( p_input, "title", VLC_VAR_CLEARCHOICES, NULL, NULL );