ssh signing: signal an error with a negative return value

ssh signing: signal an error with a negative return value

The other backend for the sign_buffer() function followed our usual
"an error is signalled with a negative return" convention, but the
SSH signer did not.  Even though we already fixed the caller that
assumed only a negative return value is an error, tighten the callee
to signal an error with a negative return as well.  This way, the
callees will be strict on what they produce, while the callers will
be lenient in what they accept.

Signed-off-by: Junio C Hamano <gitster@pobox.com>