Terminal/console need to handle invalid coordinates
chargrid_set_cursor asserted that arguments are valid yet they are passed
directly from client by terminal/console. Chargrid is probably best
positioned to validate the agruments so instead of assert, check
the arguments and don't do anything if they are invalid.
tinput was clearly forgetting about some cases where the screen could have
scrolled so simply call tinput_update_origin every time from
tinput_position_caret. This was happenning when the input line was
longer than one screen row and tab completion was invoked
(which caused assertion failure in chargrid).
