attr: fix msan issue in read_attr_from_index

Memory sanitizer (msan) is detecting a use of an uninitialized variable
(`size`) in `read_attr_from_index`:

    ==2268==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5651f3416504 in read_attr_from_index git/attr.c:868:11
    #1 0x5651f3415530 in read_attr git/attr.c
    #2 0x5651f3413d74 in bootstrap_attr_stack git/attr.c:968:6
    #3 0x5651f3413d74 in prepare_attr_stack git/attr.c:1004:2
    #4 0x5651f3413d74 in collect_some_attrs git/attr.c:1199:2
    #5 0x5651f3413144 in git_check_attr git/attr.c:1345:2
    git#6 0x5651f34728da in convert_attrs git/convert.c:1320:2
    #7 0x5651f3473425 in would_convert_to_git_filter_fd git/convert.c:1373:2
    git#8 0x5651f357a35e in index_fd git/object-file.c:2630:34
    git#9 0x5651f357aa15 in index_path git/object-file.c:2657:7
    git#10 0x5651f35db9d9 in add_to_index git/read-cache.c:766:7
    git#11 0x5651f35dc170 in add_file_to_index git/read-cache.c:799:9
    git#12 0x5651f321f9b2 in add_files git/builtin/add.c:346:7
    git#13 0x5651f321f9b2 in cmd_add git/builtin/add.c:565:18
    git#14 0x5651f321d327 in run_builtin git/git.c:474:11
    git#15 0x5651f321bc9e in handle_builtin git/git.c:729:3
    git#16 0x5651f321a792 in run_argv git/git.c:793:4
    git#17 0x5651f321a792 in cmd_main git/git.c:928:19
    git#18 0x5651f33dde1f in main git/common-main.c:62:11

The issue exists because `size` is an output parameter from
`read_blob_data_from_index`, but it's only modified if
`read_blob_data_from_index` returns non-NULL. The read of `size` when
calling `read_attr_from_buf` unconditionally may read from an
uninitialized value. `read_attr_from_buf` checks that `buf` is non-NULL
before reading from `size`, but by then it's already too late: the
uninitialized read will have happened already. Furthermore, there's no
guarantee that the compiler won't reorder things so that it checks
`size` before checking `!buf`.

Make the call to `read_attr_from_buf` conditional on `buf` being
non-NULL, ensuring that `size` is not read if it's never set.

Signed-off-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -865,7 +865,8 @@ static struct attr_stack *read_attr_from_index(struct index_state *istate,
 		stack = read_attr_from_blob(istate, &istate->cache[sparse_dir_pos]->oid, relative_path, flags);
 	} else {
 		buf = read_blob_data_from_index(istate, path, &size);
-		stack = read_attr_from_buf(buf, size, path, flags);
+		if (buf)
+			stack = read_attr_from_buf(buf, size, path, flags);
 	}
 	return stack;
 }