lib/lp-media.c: lp_media_pause () sometimes generates `Internal data stream error`

Not sure yet why and when this happens (I can't create a source that always leads to this behavior). 

I *suspect* the problem is the way we block the pads using the flag `GST_PAD_PROBE_TYPE_BLOCK_DOWNSTREAM`. I've tried to use the flag `GST_PAD_PROBE_TYPE_BLOCKING` (which seems to be more suitable), but the whole presentation stalls.