Avoid potential overflow in json_object_get_double

sscanf is always a potential problem when converting numeric
values as it does not correctly handle over- and underflow
(or at least gives no indication that it has done so).

This change converts json_object_get_double() to use strtod()
according to CERT guidelines.
Merge pull request #109 from kdopen/use_strtod

Avoid potential overflow in json_object_get_double