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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@388477 f38db490-d61c-443f-a65b-d21fe96a405b