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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@418886 f38db490-d61c-443f-a65b-d21fe96a405b