mem_heap_validate(): Compile this function also if UNIV_MEM_DEBUG is
defined.  Previously, this function was only compiled with UNIV_DEBUG.

mem_heap_free_heap_top(): Flag the memory allocated, not freed, for
Valgrind.  Otherwise, Valgrind would complain on the second call of
mem_heap_empty().

UNIV_MEM_ASSERT_RW(), UNIV_MEM_ASSERT_W(): Display additional diagnostics
for failed Valgrind checks.