Tweak mh_xmalloc(); print size in error, allocate zero bytes.

Tweak mh_xmalloc();  print size in error, allocate zero bytes.
Using `%zu' for the size_t value.  That might be a problem on older
platforms, but we'll see.  If asked for zero bytes then allocate one
rather than exit;  it's sometimes useful to allocate zero, but some
older platforms might not like it.