scalar-diagnose: avoid 32-bit overflow of size_t

scalar-diagnose: avoid 32-bit overflow of size_t

Avoid 32-bit size_t overflow when reporting the available disk space in
'get_disk_info' by casting the block size and available block count to
'off_t' before multiplying them. Without this change, 'st_mult' would
(correctly) report a size_t overflow on 32-bit systems at or exceeding 2^32
bytes of available space.

Note that 'off_t' is a 64-bit integer even on 32-bit systems due to the
inclusion of '#define _FILE_OFFSET_BITS 64' in 'git-compat-util.h' (see
b97e911643 (Support for large files on 32bit systems., 2007-02-17)).

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>