libudev: accept NULL as the argument 'struct udev*' for udev_monitor_new() or friends

As udev_monitor struct or friends are now almost independent of udev
struct. So, generating these objects without udev struct is reasonable.

src/libudev/libudev-device.c

@@ -196,8 +196,6 @@ _public_ const char *udev_device_get_property_value(struct udev_device *udev_dev
 struct udev_device *udev_device_new(struct udev *udev) {
         struct udev_device *udev_device;
 
-        assert_return_errno(udev, NULL, EINVAL);
-
         udev_device = new0(struct udev_device, 1);
         if (!udev_device) {
                 errno = ENOMEM;

src/libudev/libudev-enumerate.c

@@ -52,8 +52,6 @@ _public_ struct udev_enumerate *udev_enumerate_new(struct udev *udev) {
         _cleanup_free_ struct udev_enumerate *udev_enumerate = NULL;
         int r;
 
-        assert_return_errno(udev, NULL, EINVAL);
-
         udev_enumerate = new0(struct udev_enumerate, 1);
         if (!udev_enumerate) {
                 errno = ENOMEM;

src/libudev/libudev-hwdb.c

@@ -40,8 +40,6 @@ _public_ struct udev_hwdb *udev_hwdb_new(struct udev *udev) {
         struct udev_hwdb *hwdb;
         int r;
 
-        assert_return_errno(udev, NULL, EINVAL);
-
         r = sd_hwdb_new(&hwdb_internal);
         if (r < 0) {
                 errno = -r;

src/libudev/libudev-monitor.c

@@ -153,11 +153,6 @@ struct udev_monitor *udev_monitor_new_from_netlink_fd(struct udev *udev, const c
         struct udev_monitor *udev_monitor;
         unsigned int group;
 
-        if (udev == NULL) {
-                errno = EINVAL;
-                return NULL;
-        }
-
         if (name == NULL)
                 group = UDEV_MONITOR_NONE;
         else if (streq(name, "udev")) {

src/libudev/libudev-queue.c

@@ -45,11 +45,6 @@ _public_ struct udev_queue *udev_queue_new(struct udev *udev)
 {
         struct udev_queue *udev_queue;
 
-        if (udev == NULL) {
-                errno = EINVAL;
-                return NULL;
-        }
-
         udev_queue = new0(struct udev_queue, 1);
         if (udev_queue == NULL) {
                 errno = ENOMEM;

src/shared/udev-util.c

@@ -42,7 +42,6 @@ int udev_device_new_from_stat_rdev(struct udev *udev, const struct stat *st, str
         struct udev_device *nd;
         char type;
 
-        assert(udev);
         assert(st);
         assert(ret);
 

src/udev/udevadm-util.c

@@ -8,7 +8,6 @@ struct udev_device *find_device(struct udev *udev,
                                 const char *id,
                                 const char *prefix) {
 
-        assert(udev);
         assert(id);
 
         if (prefix && !startswith(id, prefix))