ssh: move NULL check to the free function

Let `ssh_stream_free()` take a NULL stream, as free functions should,
and remove the check from the connection setup.

The connection setup would not need the check anyhow, as we always have
a stream by the time we reach this code.

src/transports/ssh.c

@@ -177,11 +177,12 @@ static int ssh_stream_write(
 static void ssh_stream_free(git_smart_subtransport_stream *stream)
 {
 	ssh_stream *s = (ssh_stream *)stream;
-	ssh_subtransport *t = OWNING_SUBTRANSPORT(s);
-	int ret;
+	ssh_subtransport *t;
 
-	GIT_UNUSED(ret);
+	if (!stream)
+		return;
 
+	t = OWNING_SUBTRANSPORT(s);
 	t->current_stream = NULL;
 
 	if (s->channel) {
@@ -621,7 +622,6 @@ static int _git_ssh_setup_conn(
 
 done:
 	if (error < 0) {
-		if (*stream)
 		ssh_stream_free(*stream);
 
 		if (session)