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

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.
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