Fix func_config list entry allocation

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uses of ast_malloc have been replaced by ast_calloc to prevent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
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Merged revisions 400694 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@400697 65c4cc65-6c06-0410-ace0-fbb531ad65f3