test: bluetooth/tester: fix ccc_find_by_attr

Added check if ccc_values[i].attr != NULL,
before accessing to ccc_values[i].attr structure:
(handle == ccc_values[i].attr->handle)

On 20829 platform it's causing hard fault, not sure about
another platforms (in GAP testing).

Signed-off-by: Nazar Palamar <nazar.palamar@infineon.com>