libnm-glib: avoid coverity warning

libnm-glib: avoid coverity warning

  5. NetworkManager-1.0.3/libnm-glib/nm-remote-settings.c:493: var_compare_op: Comparing "error" to null implies that "error" might be null.
  8. NetworkManager-1.0.3/libnm-glib/nm-remote-settings.c:508: var_deref_op: Dereferencing null pointer "error".
  #   506|   			g_hash_table_remove (priv->pending, path);
  #   507|
  #   508|-> 		if (print_once && error->code == DBUS_GERROR_LIMITS_EXCEEDED) {
  #   509|   			g_printerr ("Warning: libnm-glib:%s(): a D-Bus limit exceeded: %s. The application might not work properly.\n"
  #   510|   			            "Consider increasing max_replies_per_connection limit in /etc/dbus-1/system.d/org.freedesktop.NetworkManager.conf "