rtpulpfecdec: Don't unset DISCONT flag on the next buffer if we just lost one

Sebastian Dröge requested to merge slomo/gst-plugins-good:fecdec into master

Consider the case where we first recover a packet (and want to unset the DISCONT flag on the next packet then because we recovered one), and then the next packet also gets lost and is not recovered. We want to keep the DISCONT flag on the packet following the unrecovered packet and not unset it. A packet is missing for real now.

CC @meh

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