check gnutls return values
There's a couple of crashes in fedora (1, 2) where it appears the problem was that the gnutls_cipher_init() failed and then on deinit or write a crash happened. While the problem seems to have been introduced through a package/dependency package update it also appears to have solved itself the same way.
Still, we should check the rv of gnutls_cipher_init and probably abort if that failed.