lightningd: initialize dev_subdaemon_fail to false.

Our testsuite uses --dev-fail-on-subdaemon-fail, so I didn't notice this
until I turned that off to chase a bug.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/lightningd.c

@@ -86,6 +86,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx,
 	ld->dev_debug_subdaemon = NULL;
 	ld->dev_disconnect_fd = -1;
 	ld->dev_hsm_seed = NULL;
+	ld->dev_subdaemon_fail = false;
 #endif
 
 	return ld;