shell: deal with weston_compositor_pick_surface() = NULL

I could crash Weston by trying to open another menu from a panel while
one menu from it was already showing.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff40a9872 in popup_grab_focus (grab=0x761968, time=4130706528, surface=0x0, x=-227, y=15) at shell.c:440
440		if (surface->resource.client == client) {
(gdb) bt
 0  0x00007ffff40a9872 in popup_grab_focus (grab=0x761968, time=4130706528, surface=0x0, x=-227, y=15) at shell.c:440
 1  0x0000000000406977 in weston_device_repick (device=0x70b4e0, time=4130706528) at compositor.c:360
 2  0x0000000000406a36 in weston_compositor_repick (compositor=0x619960) at compositor.c:382
 3  0x0000000000406ac8 in destroy_surface (resource=0x6fc6f0) at compositor.c:397
 4  0x00007ffff7bd33d8 in destroy_resource (element=0x6fc6f0, data=0x7fffffffd9fc) at wayland-server.c:355
 5  0x00007ffff7bd8d98 in for_each_helper (entries=0x757808, func=0x7ffff7bd332c <destroy_resource>, data=0x7fffffffd9fc)
    at wayland-util.c:264
 6  0x00007ffff7bd8dd4 in wl_map_for_each (map=0x757808, func=0x7ffff7bd332c <destroy_resource>, data=0x7fffffffd9fc)
    at wayland-util.c:270
 7  0x00007ffff7bd34dc in wl_client_destroy (client=0x7577d0) at wayland-server.c:385
 8  0x00007ffff7bd2e36 in wl_client_connection_data (fd=17, mask=1, data=0x7577d0) at wayland-server.c:187
 9  0x00007ffff7bd5bde in wl_event_source_fd_dispatch (source=0x74cda0, ep=0x7fffffffdae0) at event-loop.c:76
 10 0x00007ffff7bd665b in wl_event_loop_dispatch (loop=0x618900, timeout=-1) at event-loop.c:462
 11 0x00007ffff7bd42a9 in wl_display_run (display=0x6188b0) at wayland-server.c:785
 12 0x000000000040b1e1 in main (argc=1, argv=0x7fffffffdef8) at compositor.c:2182

Modify popup_grab_focus() to deal with a NULL surface.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

src/shell.c

@@ -437,7 +437,7 @@ popup_grab_focus(struct wl_grab *grab, uint32_t time,
 		container_of(grab, struct shell_surface, popup.grab);
 	struct wl_client *client = priv->surface->surface.resource.client;
 
-	if (surface->resource.client == client) {
+	if (surface && surface->resource.client == client) {
 		wl_input_device_set_pointer_focus(device, surface, time,
 						  device->x, device->y, x, y);
 		grab->focus = surface;