Fix assert for who_points_at monitor cmd for an interiorly pointed block

* fix the assert
* some better comments
* update test to verify who_points_at behaviour with an interiorly pointed block



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13609 a5019735-40e9-0310-863c-91ae7b9d1cf9