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>

contrib/scalar/scalar.c

@@ -348,7 +348,7 @@ static int get_disk_info(struct strbuf *out)
 	}
 
 	strbuf_addf(out, "Available space on '%s': ", buf.buf);
-	strbuf_humanise_bytes(out, st_mult(stat.f_bsize, stat.f_bavail));
+	strbuf_humanise_bytes(out, (off_t)stat.f_bsize * (off_t)stat.f_bavail);
 	strbuf_addf(out, " (mount flags 0x%lx)\n", stat.f_flag);
 	strbuf_release(&buf);
 #endif