libweston: explicitly cancel start_drag if no matching input device is found

Otherwise, the client will assume that dragging is in progress and remains in
that state forever.
This can happen when weston processes the mouse up event just before the
start_drag() arrives.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
3 jobs for drag-race in 2 minutes and 58 seconds (queued for 19 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #2112554
container_prep

00:00:59

 
  Build
passed #2112555
build-native-meson-default-options

00:01:59

passed #2112556
build-native-meson-no-gl-renderer

00:00:40