malloca: port to compilers that reject size-zero arrays

malloca: port to compilers that reject size-zero arrays

This fixes a bug introduced in my previous patch.
* lib/malloca.c (struct preliminary_header): Use an int
rather than a character array of size int; that's simpler.
(struct header): Remove, replacing with ...
(union header): New type.  This avoids the need for declaring a
character array of size zero, which is not allowed on some platforms.
All uses changed.