Fixed a misconception in transition from List to to ptr_vector of commit
db599f64, List::Next() returned NULL when at
end of list, mimic that.

This regression was introduced in f2153330.
As stated in PageObjectPainter::PaintPageObject:
  "The page object layouter is quite volatile."
The rest of usages of mpPageObjectLayouter looks sane, but who knows...

Symptom: segfault.
Thanks to Julien Nabet for precise pointer to problematic code.

also expose in-memory file method for unit testing.

SAL_UNUSED_PARAMETER (expanding to __attribute__ ((unused)) for GCC)
is used to annotate legitimately unused parameters, so that static
analysis tools can tell legitimately unused parameters from truly
unnecessary ones.  To that end, some patches for external modules
are also added, that are only applied when compiling with GCC and
add necessary __attribute__ ((unused)) in headers.