json.c: improve ast_json_to_ast_variables performance

When converting from a json object to an ast variables list the conversion
algorithm was doing a complete traversal of the entire variables list for
every item appended from the json structure.

This patch makes it so the list is no longer traversed for each new ast
variable being appended.

Change-Id: I8bf496a1fc449485150d6db36bfc0354934a3977