hostname-setup: do not trigger assertion when uname() is prohibited by seccomp

gethostname_full() is used in nss-myhostname, and hence random
application may indirectly call it. When an application with a too strict
seccomp filter loads the nss module, the application may trigger the
assertion.

Partially fixes #38582.

src/shared/hostname-setup.c

@@ -316,7 +316,8 @@ int gethostname_full(GetHostnameFlags flags, char **ret) {
 
         assert(ret);
 
-        assert_se(uname(&u) >= 0);
+        if (uname(&u) < 0)
+                return -errno;
 
         s = u.nodename;
         if (isempty(s) || streq(s, "(none)") ||