od: don't abort on a huge --width

[mistaste]

Summary

Fixes #12839.

od -w=<huge> aborted the process while allocating the line buffer:

$ od -w=1162652562662412622 a
memory allocation of 1162652562662412626 bytes failed
Aborted (core dumped)

Change

InputDecoder::new now computes the buffer size with a checked add and reserves
it with try_reserve_exact, returning an io::Error instead of aborting. The
caller maps it to a clean error, matching the graceful out-of-memory behavior of
GNU od:

$ od -w=1162652562662412622 a
od: memory exhausted

A width that does not fit in usize is still rejected by the existing
"argument too large" check.

Checks

• New regression test test_huge_width_argument; existing od width tests and the input_decoder unit test still pass.
• cargo fmt clean, cargo clippy -p uu_od --all-targets clean.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.