shell: fix uninitialized variable warning

shell: fix uninitialized variable warning

and shut-up valgrind:
 Conditional jump or move depends on uninitialised value(s)
    at 0xB5AFB05: shell_surface_configure (shell.c:2162)
    by 0x407B0C: surface_attach (compositor.c:1225)
    by 0x621FA13: ffi_call_unix64
    by 0x621F434: ffi_call
    by 0x4E3D3F5: wl_closure_invoke (connection.c:758)
    by 0x4E3786C: wl_client_connection_data (wayland-server.c:255)
    by 0x4E3AA19: wl_event_source_fd_dispatch (event-loop.c:78)
    by 0x4E3B533: wl_event_loop_dispatch (event-loop.c:460)
    by 0x4E38D2C: wl_display_run (wayland-server.c:907)
    by 0x40B5DD: main (compositor.c:2748)

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>