devices: remove unneeded pointer check

src/core/devices/nm-lldp-listener.c:911: check_after_deref:
  Null-checking "self" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.

Fixes: 04e72b6b4d ('lldp: use new libnm-lldp instead of systemd's sd_lldp_rx')

src/core/devices/nm-lldp-listener.c

@@ -908,8 +908,7 @@ nm_lldp_listener_new(int                  ifindex,
     return self;
 
 fail:
-    if (self)
-        nm_g_slice_free(self);
+    nm_g_slice_free(self);
     return NULL;
 }