wl_array: Set data to invalid address after free

wl_array: Set data to invalid address after free

Explicitly set the data member to an invalid memory address during
wl_array_release, such that re-using a freed wl_array without re-initializing
causes a crash. In addition, this pointer assignment makes wl_array_release
testable.

Define a constant for the invalid memory address, and add documentation about
this behavior, starting at libwayland version 1.13.

See https://lists.freedesktop.org/archives/wayland-devel/2016-September/031116.html

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
[Pekka: remove the doc about crashing]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>