bindings: python: fix uninitialized default_vals being passed to gpiod_LineBulk_request()

bindings: python: fix uninitialized default_vals being passed to gpiod_LineBulk_request()

If "default_vals" is not provided in the kwds then default_vals are
passed uninitialized to gpiod_line_request_bulk(), so rename the
existing default_vals to vals and introduce a new default_vals that
points to vals, or NULL if no defaults have been passed.

Fixes: 96c524c4951c (bindings: implement python bindings)
Reported-by: Pedro Botella <pbotella@gmail.com>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>