When the "freeswitch" package was installed directly on Debian, i.e.
without one of the meta packages pulling in the vanilla configuration
(freeswitch-conf-vanilla), the "postinst" script failed:

  $ apt-get install freeswitch
  …
  Setting up freeswitch (1.6.6~13~d2d0b328-1~jessie+1) ...
  cp: cannot stat ‘/usr/share/freeswitch/conf/vanilla/*’: No such file
  or directory
  …

With this change the code tests for "freeswitch.xml" before attempting
to copy the configuration files.