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~d2d0b32-1~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.
