util_sscanf_int treats empty string as 0 without errors

[eivindjahren]

util_sscanf_int("", &value) returns ok, with value set to 0. This is has the effect that

from resdata import ResdataUtil

def test_failure():
    data_contents = "PARALLEL '' /\n"
    with open("TEST.DATA", "w") as f:
        f.write(data_contents)
    print(ResdataUtil.get_num_cpu("TEST.DATA"))

prints 0 rather than 1.