ffmemless: Check for null file_name parameter

ffm_read_props segfaults if null file_name parameter is passed.
Added a check to return NULL in this case instead of crashing.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>
ffmemless: Get system settings file from env

Changed the system wide override settings file path
to come from FF_MEMLESS_SETTINGS evironment variable instead
of configured file path.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>
Merge pull request #9 from kjokinie/master

ffmemless: Change system settings file to env
core: Add optional-plugins option for plugins.

Adds possibility to add plugins whose loading may
fail and ngfd still continues startup.