app_meetme: Fix channels lingering when hung up under certain conditions

Channels would get stuck and MeetMe would repeatedly display an Unable
to write frame to channel error in the conf_run function if hung up
during certain sound prompts such as during user count announcements.
This patch fixes that by reintroducing a hangup check in the meetme's
main loop (also in conf_run).

(closes issue ASTERISK-20486)
Reported by: Michael Cargile
Review: https://reviewboard.asterisk.org/r/2187/
Patches:
    meetme_hangup_patch_ASTERISK-20486_v3.diff uploaded by Jonathan Rose (license 6182)
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Merged revisions 376307 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 376308 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 376310 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376312 65c4cc65-6c06-0410-ace0-fbb531ad65f3