base-manager: ensure all GUdevDevices have subsystem and name set

Under certain rare conditions (e.g. race between querying devices of a
given subsystem and the kernel tearing those devices down), the
subsystem reported for a GUdevDevice seems to be NULL.

So, ensure both subsystem and name are set on the GUdevDevice before we
process them.

The issue has been observed on GUdevDevices listed by
g_udev_client_query_by_subsystem(), not on the ones asynchronously
reported via uevents, but we add the validity check on both places for
consistency.

Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/343

src/mm-base-manager.c

@@ -473,6 +473,17 @@ handle_uevent (MMBaseManager *self,
                const gchar   *action,
                GUdevDevice   *device)
 {
+    const gchar *subsystem;
+    const gchar *name;
+
+    subsystem = g_udev_device_get_subsystem (device);
+    name      = g_udev_device_get_name      (device);
+
+    /* Valid udev devices must have subsystem and name set; if they don't have
+     * both things, we silently ignore them. */
+    if (!subsystem || !name)
+        return;
+
     if (g_str_equal (action, "add") || g_str_equal (action, "move") || g_str_equal (action, "change")) {
         g_autoptr(MMKernelDevice) kernel_device = NULL;
 
@@ -482,7 +493,7 @@ handle_uevent (MMBaseManager *self,
     }
 
     if (g_str_equal (action, "remove")) {
-        device_removed (self, g_udev_device_get_subsystem (device), g_udev_device_get_name (device));
+        device_removed (self, subsystem, name);
         return;
     }
 }
@@ -496,11 +507,20 @@ typedef struct {
 static gboolean
 start_device_added_idle (StartDeviceAdded *ctx)
 {
-    MMKernelDevice *kernel_device;
+    const gchar *subsystem;
+    const gchar *name;
+
+    subsystem = g_udev_device_get_subsystem (ctx->device);
+    name      = g_udev_device_get_name      (ctx->device);
+
+    /* Valid udev devices must have subsystem and name set; if they don't have
+     * both things, we silently ignore them. */
+    if (subsystem && name) {
+        g_autoptr(MMKernelDevice) kernel_device = NULL;
 
         kernel_device = mm_kernel_device_udev_new (ctx->self->priv->udev, ctx->device);
         device_added (ctx->self, kernel_device, FALSE, ctx->manual_scan);
-    g_object_unref (kernel_device);
+    }
 
     g_object_unref (ctx->self);
     g_object_unref (ctx->device);