Ignore recovered zero-length secondary UDPTL packets

Ignore recovered zero-length secondary UDPTL packets

In some cases, recovering lost packets using the secondary packet
recovery mechanism with UDPTL/T.38 can result in the recovery of
zero-length packets. These must be ignored or the frame generated from
them can cause segfaults and allocation failures.

(closes issue ASTERISK-19762)
(closes issue ASTERISK-19373)
Reported-by: Benjamin (bulkorok)
Reported-by: Rob Gagnon (rgagnon)
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Merged revisions 371544 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371545 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371546 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371547 65c4cc65-6c06-0410-ace0-fbb531ad65f3