view_get_title: add NULL check

Otherwise, we'll segfault:

0x00007ffff7c36715 in __strlen_avx2 () at /usr/lib/libc.so.6
0x000055555555ae00 in view_get_title (view=0x5555558403c0) at ../view.c:34

view.c

@@ -28,6 +28,9 @@ char *
 view_get_title(struct cg_view *view)
 {
 	const char *title = view->impl->get_title(view);
+	if (!title) {
+		return NULL;
+	}
 	return strndup(title, strlen(title));
 }