compiler.h: Introduce caa_container_of_check_null

The approach taken by caa_unqual_scalar_typeof requires use of _Generic
which requires full C11 support. Currently liburcu supports C99.
Therefore, this approach is not appropriate for now.

Instead, introduce caa_container_of_check_null which returns NULL if the
ptr is NULL before offsetting by the member offset.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I0ac1cacc67d83bd3dad6fb6cd2e6595190735441