touchpad: never reduce the slot count to 0

touchpad: never reduce the slot count to 0

Where a user releases all touches during a SYN_DROPPED and then puts more than
one finger back down before we sync, we end up with nonzero fake touches but
a zero slot count. This is caused by a wrong event sequences provided by
libevdev in that case.

This really needs to be fixed in libevdev, see
https://gitlab.freedesktop.org/libevdev/libevdev/merge_requests/19

In the meantime, put a check in to ignore that case and never reduce the slot
count to 0. It still leaves us open for some issues where 3fg gestures may
stop working if the right sequences are triggered during SYN_DROPPED but
updating libevdev will eventually make that go away too.

Fixes #422

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