od: fix some unlikely integer overflows

od: fix some unlikely integer overflows

* src/od.c (print_n_spaces, pad_at, pad_at_overflow):
New static functions.
(struct tspec, PRINT_FIELDS, print_named_ascii, print_ascii)
(decode_one_format, write_block, main):
Use idx_t, not int, for counts that depend on the number
of bytes in an object.
(decode_one_format): Use print_n_spaces to output spaces.
(PRINT_FIELDS, print_named_ascii, print_ascii):
Use pad_at to avoid integer overflow.
(write_block): Do not use %*s to pad, as the total pad might
exceed INT_MAX.  Instead, pad by hand with putchar (' ').
(main): Use pad_at_overflow to report integer overflow due to
oversize -w.  Use better way to tell whether -w is used,
without needing IF_LINT.
* tests/od/big-w.sh: New test.
* tests/local.mk (all_tests): Add it.