Fix mempool2 test

Fix mempool2 test
mempool2 test was using a wrong address for the pool arguments
in the client requests VALGRIND_MEMPOOL_FREE.
So, instead of testing what it was supposed to test (e.g. reading
memory from a freed mempool block), this was all causing
an illegal memory pool address.
So, fix the calls, and add a specific test to verify the illegal
mempool address.

(detected while looking at bug 375415)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16369