uORB: fix node_open: *instance is read even though it's an output parameter

This fixes a subtle bug: the instance parameter of orb_advertise is an output
parameter and thus its value can be random. However in node_open this value
is accessed and thus the open(...) call could succeed even though it should
not. This can happen for example if a second advertiser of a topic calls
orb_advertise_multi with *instance=0.

The existing implementation worked only because *instance was initialized
with -1 in most cases.

src/modules/uORB/uORBMain.cpp

@@ -94,6 +94,10 @@ uorb_main(int argc, char *argv[])
 	 * Test the driver/device.
 	 */
 	if (!strcmp(argv[1], "test")) {
+		if (!g_dev) {
+			PX4_WARN("orb is not running! start it first");
+			return -ESRCH;
+		}
 		uORBTest::UnitTest &t = uORBTest::UnitTest::instance();
 		return t.test();
 	}