ptx: report that -g/-w negatve_num as invalid

[oech3]

Closes #14315

No need to remove clap just for reporting -g -5 as (invalid) value:

> target/debug/ptx -g -5 f
error: invalid value '-5' for '--gap-size <NUMBER>': invalid digit found in string

For more information, try '--help'.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Congrats! The gnu test tests/pr/bounded-memory is now passing!