wqueue: check before dereferencing

This was found by coverity.

src/platforms/common/px4_work_queue/test/wqueue_start.cpp

@@ -64,7 +64,7 @@ int wqueue_test_main(int argc, char *argv[])
 						 SCHED_PRIORITY_MAX - 5,
 						 2000,
 						 PX4_MAIN,
-						 (argv) ? (char *const *)&argv[2] : (char *const *)nullptr);
+						 (argv[2]) ? (char *const *)&argv[2] : (char *const *)nullptr);
 
 		return 0;
 	}