sdpdemux: ensure that only one srcpad is created per stream

sdpdemux: ensure that only one srcpad is created per stream

If two senders use the same multicast IP and port then new_session_pad()
may try to add a srcpad to the same stream twice.

stream->srcpad is updated but gst_element_add_pad() fails the second
time. As a result stream->srcpad points to a deleted object and
access in gst_sdp_demux_stream_free() fails with a segfault.

Just ignore the second pad. Nothing useful can be done with it anyway.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4873>