Fix pin parameter behavior regression in MeetMe

The last time this code was touched (by me), a subtlety was missed based on the
difference between needing to check a pin's validity and the need to prompt
for a pin.

(closes issue ASTERISK-18488)
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Merged revisions 344102 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 344103 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@344104 65c4cc65-6c06-0410-ace0-fbb531ad65f3