GSM: Fix encoding problems with GSM

(closes issue ASTERISK-20457)
Reported by: Richard Miller
Patches:
	code.patch uploaded by Richard Miller (license 5685)
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Merged revisions 375272 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 375273 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@375288 65c4cc65-6c06-0410-ace0-fbb531ad65f3