bindings: cxx: do not initialize a chip's shared_ptr with nullptr

bindings: cxx: do not initialize a chip's shared_ptr with nullptr

A shared_ptr initialized with nullptr will still call the deleter which
causes a NULL-pointer dereference if we create a chip_iter when there
are no GPIO chips in the system.

Only initialize the current chip object in chip_iter if
gpiod_chip_iter_next_noclose() returned a valid pointer.

Signed-off-by: Patrick Boettcher <p@yai.se>
[Bartosz:
  - tweaked the commit message
  - use std::move when assigning the chip object]
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>