d/icecast2.postinst: fix failure on missing env file

If /etc/default/icecast2 is missing, the command substitution fails and as of set -e leads to an upgrade failure. Everywhere else /etc/default/icecast2 is handled as optional, which makes sense for an environment file.

This change moves the command substitution into the conditional, to be called only if the file exists, and not abort the script in any case.

Signed-off-by: MichaIng micha@dietpi.com