UNIV_MEM_ASSERT_RW(): New macro, to check that the contents of a memory
area is defined.
UNIV_MEM_ASSERT_W(): New macro, to check that a memory area is writable.
UNIV_MEM_ASSERT_AND_FREE(): New macro, to check that the memory is
writable before declaring it free (unwritable). This replaces UNIV_MEM_FREE()
in many places.
mem_init_buf(): Declare the memory undefined.
mem_erase_buf(): Declare the memory freed.