Make ParkAndAnnounce return to priority + 1 when return context is not defined

The ParkAndAnnounce application documentation for the optional return_context
parameter states the following:

return_context
    The goto-style label to jump the call back into after timeout. Default
    'priority+1'.

Unfortunately, the application was sending the channel back into the dialplan
at 'priority', which is the ParkAndAnnounce application call. This causes an
infinite loop of the channel constantly being parked, announced, timed out,
parked, announced, timed out... while fun, especially for those callers you
wish to drive to the end of madness, this was not the intent of the
application.

(closes issue ASTERISK-20113)
Reported by: serginuez
patches:
  app_parkandannounce.diff uploaded by serginuez (License 6405)
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Merged revisions 381916 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 381917 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381918 65c4cc65-6c06-0410-ace0-fbb531ad65f3