generator: Exit if there's no /run/ostree

Currently if run in a container image under systemd, we will
incorrectly synthesize a `var.mount` unit even if `ostree-prepare-root`
hasn't run.

The comment here said why we didn't do that before, but that's
for the really legacy embedded-only "ostree-prepare-root-static"
path, and even then I'm pretty sure it was wrong because
the generator here only runs in the *real* root, and we should
have `/run/ostree` at that point.

src/libostree/ostree-impl-system-generator.c

@@ -251,18 +251,18 @@ _ostree_impl_system_generator (const char *normal_dir, const char *early_dir, co
   if (unlinkat (AT_FDCWD, INITRAMFS_MOUNT_VAR, 0) == 0)
     return TRUE;
 
+  // If we're not booted via ostree, do nothing
+  if (!glnx_fstatat_allow_noent (AT_FDCWD, OTCORE_RUN_OSTREE, NULL, 0, error))
+    return FALSE;
+  if (errno == ENOENT)
+    return TRUE;
+
   g_autofree char *cmdline = read_proc_cmdline ();
   if (!cmdline)
     return glnx_throw (error, "Failed to read /proc/cmdline");
-
-  /* If we're installed on a system which isn't using OSTree for boot (e.g.
-   * package installed as a dependency for flatpak or whatever), silently
-   * exit so that we don't error, but at the same time work where switchroot
-   * is PID 1 (and so hasn't created /run/ostree-booted).
-   */
   g_autofree char *ostree_cmdline = otcore_find_proc_cmdline_key (cmdline, "ostree");
-  if (!ostree_cmdline)
-    return TRUE;
+  // SAFETY: If we have /run/ostree, then we must have the ostree= karg
+  g_assert (ostree_cmdline);
 
   if (!require_internal_units (normal_dir, early_dir, late_dir, error))
     return FALSE;