sdpdemux: ensure that only one srcpad is created per stream

Michael Olbrich requested to merge mol/gstreamer:submit/bad-sdp into main

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 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.

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