json_pointer.c: initialize idx

json_pointer.c: initialize idx

Fix the following build failure with gcc 5:

/home/thomas/autobuild/instance-2/output-1/build/json-c-0.17/json_pointer.c: In function 'json_pointer_result_get_recursive':
/home/thomas/autobuild/instance-2/output-1/build/json-c-0.17/json_pointer.c:193:25: error: 'idx' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    res->index_in_parent = idx;
                         ^

Fixes:
 - http://autobuild.buildroot.org/results/523b35a979d59121fe4e18c38171792b06233940/

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>