Fix SendText AMI action to never return non-zero.

AMI actions must never return non-zero unless they intend to close the AMI
connection.  (Which is almost never.)

(closes issue ASTERISK-21779)
Reported by: Paul Goldbaum
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Merged revisions 388477 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 388478 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388479 65c4cc65-6c06-0410-ace0-fbb531ad65f3