Fix valgrind found memcpy issues in codec_ilbc.

Valgrind found codec_ilbc using memcpy instead of memmove for overlapping
memory blocks.

(issue ASTERISK-19890)
(closes issue ASTERISK-20231)
Reported by: Walter Doekes
Patches:
      ASTERISK-20231.patch (license #5674) patch uploaded by Walter Doekes
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Merged revisions 373640 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373645 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373650 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373656 65c4cc65-6c06-0410-ace0-fbb531ad65f3