static-delta: Validate decompressed size against declared usize

static-delta: Validate decompressed size against declared usize

Each delta part header already declares the expected uncompressed
size of the resulting objects.  Pass that through to
_ostree_static_delta_part_open() and use it as the decompression
limit when it is smaller than the hard cap.  This catches a
crafted delta that declares a small usize (passing the free-space
pre-check) but actually decompresses to something much larger.

The hard cap (512 MiB) remains as a backstop when the declared
size is unavailable (e.g. the show/dump path passes 0).

Suggested-by: Colin Walters <walters@verbum.org>