Free ast_str objects when temp file fails to be created in MiniVM

The previous commit (r372554) was from a patch that was written before
r366880, which ensured that ast_str objects allocated in the sendmail
routine were free'd in off nominal paths.  This commit frees the
string objects in the off nominal path introduced in r372554.

(issue ASTERISK-17133)
Reported by: Tzafrir Cohen
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Merged revisions 372581 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372582 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@372583 65c4cc65-6c06-0410-ace0-fbb531ad65f3