Updated json-c to the latest version 0.13.1.
- Bump the major version of the .so library generated up to 4.0 to avoid
conflicts because some downstream packagers of json-c had already done
their own bump to ".so.3" for a much older 0.12 release.
- Add const size_t json_c_object_sizeof()
- Avoid invalid free (and thus a segfault) when ref_count gets < 0
- PR#394: fix handling of custom double formats that include a ".0"
- Avoid uninitialized variable warnings in json_object_object_foreach
- Issue #396: fix build for certain uClibc based systems.
- Add a top level fuzz directory for fuzzers run by OSS-Fuzz