feature_config: insure featuregroups and applicationmaps are initialized

If the features.conf is missing, the cfg->featurgroups
and cfg->applicationmaps is not initialized, resulting
in assert on ao2_find of a null container.  This patch
changes the initialization call and adds asserts for a
safeguard.

Review: https://reviewboard.asterisk.org/r/3809/
........

Merged revisions 418886 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418961 65c4cc65-6c06-0410-ace0-fbb531ad65f3