auth: Fix return status for ssh_userauth_agent()
auth: Fix return status for ssh_userauth_agent()

BUG: https://red.libssh.org/issues/201

Return SSH_AUTH_DENIED instead of SSH_AUTH_ERROR when the provided agent
offers no public keys.

Signed-off-by: Peter Volpe <3c0f0566aabf298172658453c7d6f6add8761c30@redhat.com>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

pouete/libssh,kedazo/libssh,kedazo/libssh,sebadoom/libssh,nviennot/libssh,pouete/libssh,pouete/libssh,sebadoom/libssh,sebadoom/libssh,nviennot/libssh,kedazo/libssh,nviennot/libssh,kedazo/libssh,sebadoom/libssh,pouete/libssh,nviennot/libssh