client_post_select(): Defer decrypting of server challenge.

client_post_select(): Defer decrypting of server challenge.

The old code worked only by pure luck: We allocated the crypt buffer on the stack as
an ordinary automatic variable. This buffer was filled right after we received the challenge
from the server, but it was used in a _subsequent_ call to client_post_select(). There's
no guarantee that the content of the crypt buffer stays the same between these two calls.

So defer the decryption until the client status is CL_RECEIVED_CHALLENGE, i.e. until
the socket file descriptor is known to be ready for sending back the SHA1 of the decrypted
challenge.