udptl: Correct FEC to not consider negative sequence numbers as missing

udptl: Correct FEC to not consider negative sequence numbers as missing

When using FEC, with span=3 and entries=4 Asterisk will attempt to repair
the packet with sequence number 5, as it will see that packet -4 is
missing. The result is Asterisk sending garbage packets that can kill a
fax.

This patch adds a check to see if the sequence number is valid before
checking if the packet is missing.

Review: https://reviewboard.asterisk.org/r/3657/

#ASTERISK-23908 #close
Reported by: Torrey Searle
patches:
  udptl_fec.patch uploaded by Torrey Searle (License 5334)
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Merged revisions 417318 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 417320 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@417324 65c4cc65-6c06-0410-ace0-fbb531ad65f3