avfilter_default_filter_name() is supposed to access an AVFilterContext
struct, if used with a private struct it will cause a crash since it will
access fields which are non defined in the private struct.
The function is modelled after av_default_item_name(), and will print the
name of the instance filter if defined, otherwise the name of the filter.
This allows to show the instance name in the log, which is useful when
debugging complex filter graphs.
Also add internal function ff_null_start_frame_keep_ref().
Fix crash when a following filter (e.g. settb) will unref the reference
passed by start_frame(), and then the reference is accessed in
end_frame() through inlink->cur_buf.
