Fix func_config list entry allocation

The AST_CONFIG dialplan function defined in func_config.c allocates its
config file list entries using ast_malloc. List entry allocations
destined for use with Asterisk's linked list API must be ast_calloc()d
or otherwise initialized so that list pointers are set to NULL. These
uses of ast_malloc have been replaced by ast_calloc to prevent
dereferencing of uninitialized pointer values when traversing the list.

(closes issue ASTERISK-22483)
Reported by: Brian Scott


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@400694 f38db490-d61c-443f-a65b-d21fe96a405b