evdev: fix uninitialised pointer value

evdev: fix uninitialised pointer value

==30224== Conditional jump or move depends on uninitialised value(s)
==30224==    at 0x40EE3A0: evdev_flush_motion (evdev.c:284)
==30224==    by 0x40EE6DC: evdev_input_device_data (evdev.c:352)
==30224==    by 0x4034710: wl_event_source_fd_dispatch (event-loop.c:76)
==30224==    by 0x4035171: wl_event_loop_dispatch (event-loop.c:462)
==30224==    by 0x4032F76: wl_display_run (wayland-server.c:785)
==30224==    by 0x8050972: main (compositor.c:2183)

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