core: make the library NULL-aware

core: make the library NULL-aware

Currently we almost never check the validity of pointers passed to the
core C API routines. While we cannot know if a non-NULL pointer points
to an actual object, we can at least verify that it is not NULL before
dereferencing it.

Conceptually we can think of two categories of pointers in libgpiod:

First, there are the objects we manipulate using the API. These are
normally the first arguments in any given method and they are only
instantiated inside the library and never dereferenced by the user.

They should always be valid, so passing a NULL pointer here should
always lead to a crash. Currently it will take the form of a segfault
at the moment of dereference but with this change, we make the user
program abort() with the stack trace pointing right at the offender.

The second category would be pointers to objects that are logically
parameters of methods i.e. not their first argument. Example is:
gpiod_chip_request_lines(chip, req_cfg, line_cfg) (whose logical OOP
version would be: chip->request_lines(req_cfg, line_cfg)). Here we
accept a NULL req_cfg but we'll segfault on a NULL line_cfg. For
consistency: don't crash here but instead return -1 and set errno
to EINVAL. Same everywhere else.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>