Commit 240c92dd

 added an assert
to check that the input netmask is valid. Revert that commit for
the most part, some changes to the test function are not reverted.

We don't want to assert for a valid netmask, because it's
common to read the netmask from (untrusted) user input, so we
don't want to assert against it.

The caller *could* validate the netmask from untrusted sources, but
with the assert in place it cannot validate it in the most obvious way:

    prefix = nm_utils_ip4_netmask_to_prefix (netmask);
    if (netmask != nm_utils_ip4_prefix_to_netmask (prefix))
        goto fail;

Signed-off-by: Thomas Haller <thaller@redhat.com>