gtlscertificate: Fix error reporting if a GError is not passed in
gtlscertificate: Fix error reporting if a GError is not passed in

If the certificate constructor is called as:
   g_tls_certificate_new_from_pem (data, length, NULL);
and PEM parsing fails for the private key, the function would have
continued to try and create a certificate using a NULL key_pem value,
which would have failed or crashed.

Use g_propagate_error() correctly to avoid this.

Coverity CID: 1325403

ieei/glib,ieei/glib,ieei/glib,mzabaluev/glib,ieei/glib,Distrotech/glib,cention-sany/glib,cention-sany/glib,Distrotech/glib,MathieuDuponchelle/glib,mzabaluev/glib,mzabaluev/glib,endlessm/glib,johne53/MB3Glib,Distrotech/glib,ieei/glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib,mzabaluev/glib,cention-sany/glib,Distrotech/glib,mzabaluev/glib,johne53/MB3Glib,Distrotech/glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,cention-sany/glib,cention-sany/glib,endlessm/glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib