pki_crypto: pad RSA signature blobs

Pad RSA signature blobs to the expected RSA signature length
when processing via 'pki_signature_to_blob'.

Some clients, notably PuTTY, may send unpadded RSA signatures
during the public key exchange: before this change, one can
sometimes observe failure in signature validation when using
PuTTY's 'plink' client, along these lines:

   ssh_packet_process: ssh_packet_process: Dispatching handler for packet type 50
   ssh_packet_userauth_request: ssh_packet_userauth_request: Auth request for service ssh-connection, method publickey for user 'foo'
   ssh_pki_signature_verify_blob: ssh_pki_signature_verify_blob: Going to verify a ssh-rsa type signature
   pki_signature_verify: pki_signature_verify: RSA error: error:04091077:rsa routines:INT_RSA_VERIFY:wrong signature length
   ssh_packet_userauth_request: ssh_packet_userauth_request: Received an invalid  signature from peer

For cross-reference this issue once also existed between
PuTTY and OpenSSH:

  http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/rsa-verify-failed.html

  http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-rsa.c?rev=1.19;content-type=text%2Fx-cvsweb-markup

With the fix I am unable to reproduce the above failure mode when
testing with 'plink'.