The merge commit 0fd89a1a
of commit b6ec1e8b
seems to cause occasional MemorySanitizer failures,
because it failed to replace some MEM_UNDEFINED() calls
with MEM_MAKE_ADDRESSABLE().

my_large_free(): Correctly invoke MEM_MAKE_ADDRESSABLE() after
freeing memory. Failure to do so could cause bogus
AddressSanitizer failures for memory allocated by my_large_malloc().
On MemorySanitizer, we will do nothing.

buf_pool_t::chunk_t::create(): Replace the MEM_MAKE_ADDRESSABLE()
that had been added in commit 48493132
to work around the issue.