ralloc: Fix ralloc_parent() of memory allocated out of the NULL context.

ralloc: Fix ralloc_parent() of memory allocated out of the NULL context.

If an object is allocated out of the NULL context, info->parent will be
NULL.  Using the PTR_FROM_HEADER macro would be incorrect: it would say
that ralloc_parent(ralloc_context(NULL)) == sizeof(ralloc_header).

Fixes the new "null_parent" unit test.

NOTE: This is a candidate for the 7.9, 7.10, 7.11, and 8.0 branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>