shell: fix a crash during 'make check'

$ abs_builddir=$PWD/tests gdb -args ./src/weston --module=$PWD/tests/.libs/event-test.so

(gdb) run
Starting program: /home/pq/git/wayland-demos/src/weston --module=/home/pq/git/wayland-demos/tests/.libs/event-test.so
[Thread debugging using libthread_db enabled]
Mesa: Initializing x86-64 optimizations
launching /home/pq/git/wayland-demos/tests/test-client
created output global 0x608f50
test-client: got create-surface
got surface 5 from client
got surface id 5

Program received signal SIGSEGV, Segmentation fault.
Mesa: Initializing x86-64 optimizations
0x00007fffeff72c7c in handle_pointer_focus (listener=0x74f5c0, data=0x6faa40) at shell.c:492
492		if (shsurf->unresponsive) {
(gdb) bt
 #0  0x00007fffeff72c7c in handle_pointer_focus (listener=0x74f5c0, data=0x6faa40) at shell.c:492
 #1  0x00007ffff5ed8b87 in wl_signal_emit (data=0x6faa40, signal=0x6faa88) at wayland-server.h:166
 #2  wl_pointer_set_focus (pointer=0x6faa40, surface=<optimized out>, sx=12800, sy=12800) at wayland-server.c:752
 #3  0x0000000000407d92 in weston_device_repick (seat=0x6fa930) at compositor.c:633
 #4  0x0000000000407e49 in weston_compositor_repick (compositor=0x61c510) at compositor.c:656
 #5  0x00000000004092e1 in weston_output_repaint (output=0x7b85b0, msecs=-1046834186) at compositor.c:1059
 #6  0x00000000004094b4 in weston_output_finish_frame (output=0x7b85b0, msecs=-1046834186) at compositor.c:1092
 #7  0x00007ffff211e3c1 in finish_frame_handler (data=0x7b85b0) at compositor-x11.c:284
 #8  0x00007ffff5eda603 in wl_event_source_timer_dispatch (source=0x79ee50, ep=<optimized out>) at event-loop.c:173
 #9  0x00007ffff5edaca0 in wl_event_loop_dispatch (loop=0x61b940, timeout=<optimized out>) at event-loop.c:410
 #10 0x00007ffff5ed8dbd in wl_display_run (display=0x61b8f0) at wayland-server.c:1025
 #11 0x000000000040ecb1 in main (argc=1, argv=0x7fffffffdd98) at compositor.c:3225
(gdb) print shsurf
$1 = (struct shell_surface *) 0x0

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

src/shell.c

@@ -489,7 +489,7 @@ handle_pointer_focus(struct wl_listener *listener, void *data)
 	compositor = surface->compositor;
 	shsurf = get_shell_surface(surface);
 
-	if (shsurf->unresponsive) {
+	if (shsurf && shsurf->unresponsive) {
 		set_busy_cursor(shsurf, pointer);
 	} else {
 		serial = wl_display_next_serial(compositor->wl_display);