Fixed stack overrun bug in Nettle.CBC and Nettle.Proxy.
Aka Crypto.CBC and Crypto.Buffer. This bug could cause out of C-stack by
feeding large data strings to crypt() in these classes. It's not a clear-cut
stack smash though, since the result string was allocated on the stack, and
there should be several check_c_stack calls via safe_apply before anything
gets written to it.