Ensure VALGRIND_MALLOCLIKE_BLOCK protects the red zones.

* Redzones for custom alloc were not protected by VALGRIND_MALLOCLIKE_BLOCK.
  mc_main.c client request handling completed with protection
  of the redzones.
* custom_alloc.c test modified to test this case.
* mc_errors.c modified so as to first search for a malloc-ed block
  bracketting the error : for a custom allocator, a recently freed
  block can have just been re-allocated.
  In such a case, describing the address (e.g. in case of error)
  points to the block freed rather than to the block just allocated.
  If there is *also* a recently freed block bracketting the address,
  the block description is changed to indicate that.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12439 a5019735-40e9-0310-863c-91ae7b9d1cf9
6 files changed