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.