Returns the current stream-id for the pad, or NULL if none has been
set yet, i.e. the pad has not received a stream-start event yet.
This is a convenience wrapper around Pad.getStickyEvent and
The returned stream-id string should be treated as an opaque string, its
contents should not be interpreted.
a newly-allocated copy of the stream-id for
pad, or NULL. g_free() the returned string when no longer