attr.c: move ATTR_MAX_FILE_SIZE check into read_attr_from_buf()

attr.c: move ATTR_MAX_FILE_SIZE check into read_attr_from_buf()

Commit 3c50032ff52 (attr: ignore overly large gitattributes files,
2022-12-01) added a defense-in-depth check to ensure that .gitattributes
blobs read from the index do not exceed ATTR_MAX_FILE_SIZE (100 MB).

But there were two cases added shortly after 3c50032ff52 was written
which do not apply similar protections:

  - 47cfc9bd7d0 (attr: add flag `--source` to work with tree-ish,
    2023-01-14)

  - 4723ae1007f (attr.c: read attributes in a sparse directory,
    2023-08-11) added a similar

Ensure that we refuse to process a .gitattributes blob exceeding
ATTR_MAX_FILE_SIZE when reading from either an arbitrary tree object or
a sparse directory. This is done by pushing the ATTR_MAX_FILE_SIZE check
down into the low-level `read_attr_from_buf()`.

In doing so, plug a leak in `read_attr_from_index()` where we would
accidentally leak the large buffer upon detecting it is too large to
process.

(Since `read_attr_from_buf()` handles a NULL buffer input, we can remove
a NULL check before calling it in `read_attr_from_index()` as well).

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>