Tweak mh_xrealloc(); print size on error, follow POSIX.

Tweak mh_xrealloc();  print size on error, follow POSIX.
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 free an existing
pointer, only passing it to free(3) if it's non-NULL, and then allocate
a byte so a unique pointer is returned.