pki_crypto.c: plug ecdsa_sig->[r,s] bignum leaks

Per ecdsa(3ssl), ECDSA_SIG_new does allocate its 'r' and 's' bignum fields.
Fix a bug where the initial 'r' and 's' bignums were being overwritten with
newly-allocated bignums, resulting in a memory leak.

BUG: https://red.libssh.org/issues/175

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
session: add getter for kexalgo

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
Set the correct error in ssh_options_set().

Signed-off-by: Hani Benhabiles <hani@linux.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
doc: clarify tutorial error section

Signed-off-by: Davide "FunkyAss" Del Zompo <davide.delzompo@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>