lib/video: fix invalid id in input_GetEsObjects

This fixes a SEGFAULT when calling libvlc_video_set_teletext when there is no
teletext-es.

Fixes #11854

lib/video.c

@@ -507,11 +507,14 @@ void libvlc_video_set_teletext( libvlc_media_player_t *p_mi, int i_page )
     }
 
     telx = var_GetInteger( p_input_thread, "teletext-es" );
-    if( input_GetEsObjects( p_input_thread, telx, &p_zvbi, NULL, NULL )
-        == VLC_SUCCESS )
+    if( telx >= 0 )
     {
-        var_SetInteger( p_zvbi, "vbi-page", i_page );
-        vlc_object_release( p_zvbi );
+        if( input_GetEsObjects( p_input_thread, telx, &p_zvbi, NULL, NULL )
+            == VLC_SUCCESS )
+        {
+            var_SetInteger( p_zvbi, "vbi-page", i_page );
+            vlc_object_release( p_zvbi );
+        }
     }
     vlc_object_release( p_input_thread );
 }