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>