Memory pool: allocate the block descriptor separately with the sanitizer

Memory pool: allocate the block descriptor separately with the sanitizer

Previously, njs_mp_alloc_large() embedded the block descriptor at the end of
the allocation for sizes which are not a power of two, so a read past the end
of an object stayed inside the malloc()ed region and was not detected by the
address sanitizer.

The fix is to always allocate the descriptor separately when the address
sanitizer is enabled.