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)
........

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


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@373738 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_voicemail.c

@@ -1965,6 +1965,11 @@ static void vm_imap_delete(char *file, int msgnum, struct ast_vm_user *vmu)
 		return;
 	}
 
+	if (msgnum < 0) {
+		imap_delete_old_greeting(file, vms);
+		return;
+	}
+
 	/* find real message number based on msgnum */
 	/* this may be an index into vms->msgArray based on the msgnum. */
 	messageNum = vms->msgArray[msgnum];