guile: Session record port treats premature termination as EOF.

It used to be that, while reading from a Guile "session record port" ("ports" are Guile's I/O abstraction, similar to FILE in C), an exception corresponding to GNUTLS_E_PREMATURE_TERMINATION could be thrown. This was inconvenient as users of the port may not be prepared to deal with a GnuTLS exception.

This change fixes that. I think it could go in the next 3.7 release.

Thoughts?

Ludo'.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code