fix: index handling in chomp

[wassup05]

To prevent any indices from becoming non-positive

Cause of CI failures in #999

[jalvesz]

Thanks @wassup05 for this PR, seems like this was the only procedure from the interface that is not being tested. Could you add 1 test to verify its behavior in test\string\test_string_strip_chomp.f90 ?

[perazz]

Thank you for this PR @wassup05. I agree on @jalvesz's comment to add a simple test case before this can be merged.

[wassup05]

The recent tests failed because both sides of the .and. expression are being evaluated... I am not sure if this is the standard behaviour of Fortran or compiler specific, but the next commit should fix it either way.