Fix error where improper IMAP greetings would be deleted.

(closes issue ASTERISK-20435)
Reported by: fhackenberger
Patches:
	asterisk-20435-imap-del-greeting.diff uploaded by Michael L. Young (License #5026)
	(with suggested modification made by me)
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Merged revisions 373735 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373737 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373738 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373740 65c4cc65-6c06-0410-ace0-fbb531ad65f3