Bug 975: Eliminate the use of obsolete JSON-C function.

json_object_object_get() in JSON-C library is obsolete.
json_object_object_get_ex() should be used instead.

Change-Id: I902b80df6a68f3e92f4b2511e29dfc24e2e49f3f
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>