input: Fix weston_seat_init_keyboard() error path

input: Fix weston_seat_init_keyboard() error path

The pointer seat->keyboard was set before some possible error returns.
That pointer was left unchanged in case of failure, pointing to an
uninitialized keyboard struct (that was also leaked). If a client sent
a wl_seat::get_keyboard request, that would cause Weston to crash.

Fix this by setting the seat->keyboard pointer only after the keymap
initialization is done and there is no more possibilities for failure.
Also plug the memory leaks on the error path.

https://bugs.freedesktop.org/show_bug.cgi?id=74035