od: fix various off-by-one issues with --strings with -N

od: fix various off-by-one issues with --strings with -N

* src/od.c (dump_strings): There are three related issues here
due to not accounting for the terminating NUL char appropriately.

1. Ensure BUF always has enough space for the terminating NUL.
This avoids CWE-122: Heap-based Buffer Overflow,
where we wrote a single NUL byte directly after the allocated buffer.
I.e., there should be no buffer overflow with:
  printf '%100s' | od -N100 -S1

2. Ensure we support -S == -N (END_OFFSET - STRING_MIN == ADDRESS):
I.e., there should be output with:
  printf '%100s' | od -N10 -S10

3. Ensure we always output a valid address by ensuring
the ADDRESS and I variables are kept in sync.
I.e., this should output address 0000000 not 1777777777777777777777:
  printf '%100s' | od -N10 -S1

As well as fixing these we simplify by using a single loop
to read the data, rather than two.

* doc/coreutils.texi (od invocation): Clarify that -N
implicitly NUL terminates strings.
* tests/od/od-N.sh: Add test cases.
* NEWS: Mention the bug fixes.

Fixes https://bugs.gnu.org/78880