t5319: change invalid offset for SHA-256 compatibility

t5319: change invalid offset for SHA-256 compatibility

When using SHA-1, the existing value of the byte we use is 0x13, so
writing the byte 0x07 serves to corrupt the test and verify that we
detect corruption.  However, when we use SHA-256, the value at that
offset is already 0x07, so our "corruption" doesn't work and the test
fails to detect it.

To provide a value that is truly out of range, let's use 0xff, which is
not likely to be a valid value as the high byte of a two-byte offset in
a multi-pack index this small.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>