Drop the even more home-grown alloc wrapper and use the xalloc.h
wrappers directly.

xcalloc() is added and used, because calloc() will detect integer
overflows in the size multiplication, while doing a simple
multiplication in the caller is subject to overflows which may result in
allocating not what was expected, subjecting to out-of-bounds access.

All MEM_ALLOC() calls that had a meaningful multiplication in them were
converted to xcalloc(), the rest to xzalloc().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>