attr: adjust a mismatched data type

attr: adjust a mismatched data type

On platforms where `size_t` does not have the same width as `unsigned
long`, passing a pointer to the former when a pointer to the latter is
expected can lead to problems.

Windows and 32-bit Linux are among the affected platforms.

In this instance, we want to store the size of the blob that was read in
that variable. However, `read_blob_data_from_index()` passes that
pointer to `read_object_file()` which expects an `unsigned long *`.
Which means that on affected platforms, the variable is not fully
populated and part of its value is left uninitialized. (On Big-Endian
platforms, this problem would be even worse.)

The consequence is that depending on the uninitialized memory's
contents, we may erroneously reject perfectly fine attributes.

Let's address this by passing a pointer to a variable of the expected
data type.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>