Fewer uses of size_t in buffer.c

Fewer uses of size_t in buffer.c

* src/buffer.c (flush_write_ptr, flush_bufmap, bufmap_locate):
(struct zip_magic, available_space_after, _flush_write)
(short_read, flush_archive, try_new_volume)
(gnu_add_multi_volume_header, simple_flush_read)
(simple_flush_write, _gnu_flush_read, _gnu_flush_write)
(gnu_flush_write): Prefer idx_t to size_t when either will do, as
signed types are typically safer.  For a tiny value in memory,
just use ‘char’.