rt-tests: deadline_test: Add NULL check before freeing setcpu_buf

setcpu_buf doesn't always call malloc, so NULL check it before freeing

Signed-off-by: John Kacur <jkacur@redhat.com>

src/sched_deadline/deadline_test.c

@@ -2092,6 +2092,8 @@ int main (int argc, char **argv)
 		printf("\n");
 	}
 
-	free(setcpu_buf);
+	if (!setcpu_buf)
+		free(setcpu_buf);
+
 	return 0;
 }