Flow is so this cannot be unset, we'd abort if we never get an event. The
compiler doesn't know that though.

In file included from tablet.c:35:0:
tablet.c: In function ‘motion’:
litest.h:202:45: warning: ‘last_reported_y’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
  ck_assert_int_lt((int)(a_ * 256), (int)(b_ * 256))
                                             ^
tablet.c:158:26: note: ‘last_reported_y’ was declared here
  double last_reported_x, last_reported_y;
                          ^
In file included from tablet.c:35:0:
litest.h:208:45: warning: ‘last_reported_x’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
  ck_assert_int_gt((int)(a_ * 256), (int)(b_ * 256))
                                             ^
tablet.c:158:9: note: ‘last_reported_x’ was declared here
  double last_reported_x, last_reported_y;

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>