remote: use the library's certificate validity if no callback is set

We should not return 0, as in this case that means we let it through,
return an appropriate error instead.

remote.go

@@ -160,8 +160,13 @@ func updateTipsCallback(_refname *C.char, _a *C.git_oid, _b *C.git_oid, data uns
 //export certificateCheckCallback
 func certificateCheckCallback(_cert *C.git_cert, _valid C.int, _host *C.char, data unsafe.Pointer) int {
 	callbacks := (*RemoteCallbacks)(data)
+	// if there's no callback set, we need to make sure we fail if the library didn't consider this cert valid
 	if callbacks.CertificateCheckCallback == nil {
-		return 0
+		if _valid == 1 {
+			return 0
+		} else {
+			return C.GIT_ECERTIFICATE
+		}
 	}
 	host := C.GoString(_host)
 	valid := _valid != 0